exceptions.md

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Exceptions

Definition

In computing and computer programming, exception handling is the process of responding to the occurrence of exceptions – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an exception breaks the normal flow of execution and executes a pre-registered exception handler; the details of how this is done depend on whether it is a hardware or software exception and how the software exception is implemented.

Exceptions are defined by different layers of a computer system, and the typical layers are CPU-defined interrupts, operating system (OS)-defined signals, programming language-defined exceptions. Each layer requires different ways of exception handling although they may be interrelated, e.g. a CPU interrupt could be turned into an OS signal. Some exceptions, especially hardware ones, may be handled so gracefully that execution can resume where it was interrupted.

The definition of an exception is based on the observation that each procedure has a precondition, a set of circumstances for which it will terminate "normally". An exception handling mechanism allows the procedure to raise an exception if this precondition is violated, for example if the procedure has been called on an abnormal set of arguments. The exception handling mechanism then handles the exception.

The precondition, and the definition of exception, is subjective. The set of "normal" circumstances is defined entirely by the programmer, e.g. the programmer may deem division by zero to be undefined, hence an exception, or devise some behavior such as returning zero or a special "ZERO DIVIDE" value (circumventing the need for exceptions). Common exceptions include an invalid argument (e.g. value is outside of the domain of a function), an unavailable resource (like a missing file, a network drive error, or out-of-memory errors), or that the routine has detected a normal condition that requires special handling, e.g., attention, end of file. Social pressure is a major influence on the scope of exceptions and use of exception-handling mechanisms, i.e. "examples of use, typically found in core libraries, and code examples in technical books, magazine articles, and online discussion forums, and in an organization’s code standards".

Exception handling solves the semipredicate problem, in that the mechanism distinguishes normal return values from erroneous ones. In languages without built-in exception handling such as C, routines would need to signal the error in some other way, such as the common return code and errno pattern. Taking a broad view, errors can be considered to be a proper subset of exceptions, and explicit error mechanisms such as errno can be considered (verbose) forms of exception handling. The term "exception" is preferred to "error" because it does not imply that anything is wrong - a condition viewed as an error by one procedure or programmer may not be viewed that way by another.

-- Wikipedia

Description

PHP has an exception model similar to that of other programming languages. An exception can be thrown, and caught ("catched") within PHP. Code may be surrounded in a try block, to facilitate the catching of potential exceptions. Each try must have at least one corresponding catch or finally block.

If an exception is thrown and its current function scope has no catch block, the exception will "bubble up" the call stack to the calling function until it finds a matching catch block. All finally blocks it encounters along the way will be executed. If the call stack is unwound all the way to the global scope without encountering a matching catch block, the program will terminate with a fatal error unless a global exception handler has been set.

The thrown object must be an instance of Throwable. Trying to throw an object that is not will result in a PHP fatal error.

-- PHP Reference

Example: Exception

<?php

class SomeException extends Exception
{
    public mixed $value;

    function __construct(mixed $value)
    {
        $this->value = $value;
        $this->message = "Value has beign given.";
    }
}

class OtherException extends SomeException
{
    function __construct(int $number)
    {
        $this->value = $number;
        $this->message = "Number has beign given.";
    }
}

function someRiskySituation(): void
{
    $input = readline("Input: ");

    if (empty($input)) {
        print("No exception.\n");
    } elseif (!is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

try {
    someRiskySituation();
} catch (OtherException $e) {
    print("Some exception: " . $e->getMessage() . " (" . $e->value . ")\n");
} catch (SomeException $e) {
    print("Other exception: " . $e->getMessage() . " (" . $e->value . ")\n");
} finally {
    print("Will always execute.\n");
}

print("Will also execute (due to exception has been catched).\n");

Result (PHP 8.4):

Input:
No exception.
Will always execute.
Will also execute (due to exception has been catched).

Input: 1
Some exception: Number has beign given. (1)
Will always execute.
Will also execute (due to exception has been catched).

Input: a
Other exception: Value has beign given. (a)
Will always execute.
Will also execute (due to exception has been catched).

Source code: Example

Throwing exception

throw expression

Example: Throwing an exception

<?php
function inverse($x) {
    if (!$x) {
        throw new Exception('Division by zero.');
    }
    return 1/$x;
}

try {
    echo inverse(5) . "\n";
    echo inverse(0) . "\n";
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
}

// Continue execution
echo "Hello World\n";
?>

The above example will output:

0.2
Caught exception: Division by zero.
Hello World

-- PHP Reference

Example: Throwing an exception

<?php

throw new Exception('The exception has been thrown.');

Result (PHP 8.4):

PHP Fatal error:  Uncaught Exception: The exception has been thrown. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_exception.php:3
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_exception.php on line 3

Source code: Example

Example: Throwing an defined exception

<?php

class SomeException extends Exception
{
    public string $name = 'Some exception';

    function __construct()
    {
        $this->message = 'The exception has been thrown.';
    }
}

throw new SomeException();

Result (PHP 8.4):

PHP Fatal error:  Uncaught SomeException: The exception has been thrown. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_defined_exception.php:13
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_defined_exception.php on line 13

Source code: Example

As of PHP 8.0.0, the throw keyword is an expression and may be used in any expression context. In prior versions it was a statement and was required to be on its own line.

-- PHP Reference

Example: Throw as an expression

Only permitted in PHP 8.0.0 and later.

<?php

function test() {
    do_something_risky() or throw new Exception('It did not work');
}

function do_something_risky() {
    return false; // Simulate failure
}

try {
    test();
} catch (Exception $e) {
    print $e->getMessage();
}
?>

The above example will output:

It did not work

-- PHP Reference

Example: Throwing an exception as an expression

<?php

readline("Give some number: ") or throw new Exception('The exception has been thrown.');

Result (PHP 8.4):

Give some number: 0
PHP Fatal error:  Uncaught Exception: The exception has been thrown. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_exception_as_expression.php:3
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/throwing_exception_as_expression.php on line 3

Give some number: 1

Source code: Example

Catching exception

catch block

A catch block defines how to respond to a thrown exception. A catch block defines one or more types of exception or error it can handle, and optionally a variable to which to assign the exception. (The variable was required prior to PHP 8.0.0.) The first catch block a thrown exception or error encounters that matches the type of the thrown object will handle the object.

When an exception is thrown, code following the statement will not be executed, and PHP will attempt to find the first matching catch block. If an exception is not caught, a PHP fatal error will be issued with an "Uncaught Exception ..." message, unless a handler has been defined with set_exception_handler().

-- PHP Reference

Example: Catching an exception

<?php

try {
    throw new Exception('The exception has been thrown.');
} catch (Exception $exception) {
    print("The exception has been catched:\n");
    print_r($exception);
}

Result (PHP 8.4):

The exception has been catched:
Exception Object
(
    [message:protected] => The exception has been thrown.
    [string:Exception:private] =>
    [code:protected] => 0
    [file:protected] => /projects/php.lab/examples/code/errors_and_exceptions/exceptions/catching_exception.php
    [line:protected] => 4
    [trace:Exception:private] => Array
        (
        )

    [previous:Exception:private] =>
)

Source code: Example

Example: Catching a defined exception

<?php

class SomeException extends Exception
{
    public string $name = 'Some exception';

    function __construct()
    {
        $this->message = 'The exception has been thrown.';
    }
}

try {
    throw new SomeException();
} catch (Exception $exception) {
    print("The exception has been catched:\n");
    print_r($exception);
}

Result (PHP 8.4):

The exception has been catched:
SomeException Object
(
    [message:protected] => The exception has been thrown.
    [string:Exception:private] =>
    [code:protected] => 0
    [file:protected] => /projects/php.lab/examples/code/errors_and_exceptions/exceptions/catching_defined_exception.php
    [line:protected] => 14
    [trace:Exception:private] => Array
        (
        )

    [previous:Exception:private] =>
    [name] => Some exception
)

Source code: Example

As of PHP 8.0.0, the variable name for a caught exception is optional. If not specified, the catch block will still execute but will not have access to the thrown object.

-- PHP Reference

Example: Omitting the caught variable

Only permitted in PHP 8.0.0 and later.

<?php

class SpecificException extends Exception {}

function test() {
    throw new SpecificException('Oopsie');
}

try {
    test();
} catch (SpecificException) {
    print "A SpecificException was thrown, but we don't care about the details.";
}
?>

The above example will output:

A SpecificException was thrown, but we don't care about the details.

-- PHP Reference

Example: Catching an exception and ommiting the caught variable

<?php

try {
    throw new Exception('The exception has been thrown.');
} catch (Exception) {
    print("The exception has been catched.\n");
}

Result (PHP 8.4):

The exception has been catched.

Source code: Example

Multiple catch blocks can be used to catch different classes of exceptions. Normal execution (when no exception is thrown within the try block) will continue after that last catch block defined in sequence. Exceptions can be thrown (or re-thrown) within a catch block. If not, execution will continue after the catch block that was triggered.

-- PHP Reference

Example: Catching exceptions of various types

<?php

class NumberValueException extends Exception
{
    public int $number;
}

class ZeroException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "0 number has beign given.";
    }
}

class OneException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1 number has beign given.";
    }
}

class ThousandException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1000 number has beign given.";
    }
}

function drawNumber()
{
    $number = readline("Give some number: ");

    if ($number == 0) {
        throw new ZeroException($number);
    } elseif ($number == 1) {
        throw new OneException($number);
    } elseif ($number == 1000) {
        throw new ThousandException($number);
    } elseif ($number == 10000) {
        throw new NumberValueException();
    }

    return $number;
}

print("Program begin...\n");

try {
    print("Risky code...\n");

    $number = drawNumber();

    print("Given number " . $number . " didn't case exception throwing.\n");
} catch (ZeroException $e) {
    print("CASE 1: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (OneException $e) {
    print("CASE 2: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (ThousandException $e) {
    print("CASE 3: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (Exception) {
    print("Exception of unknown type catched.\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Give some number:
Given number  didn't case exception throwing.
Program end...

Program begin...
Risky code...
Give some number: 0
CASE 1: 0 number has beign given. (0)
Program end...

Program begin...
Risky code...
Give some number: 1
CASE 2: 1 number has beign given. (1)
Program end...

Program begin...
Risky code...
Give some number: 2
Given number 2 didn't case exception throwing.
Program end...

Program begin...
Risky code...
Give some number: 1000
CASE 3: 1000 number has beign given. (1000)
Program end...

Program begin...
Risky code...
Give some number: 10000
Exception of unknown type catched.
Program end...

Source code: Example

As of PHP 7.1.0, a catch block may specify multiple exceptions using the pipe (|) character. This is useful for when different exceptions from different class hierarchies are handled the same.

-- PHP Reference

Example: Multi catch exception handling

<?php

class MyException extends Exception { }

class MyOtherException extends Exception { }

class Test {
    public function testing() {
        try {
            throw new MyException();
        } catch (MyException | MyOtherException $e) {
            var_dump(get_class($e));
        }
    }
}

$foo = new Test;
$foo->testing();

?>

The above example will output:

string(11) "MyException"

-- PHP Reference

Example: Catching an exception of multiple types

<?php

class ZeroException extends Exception
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "0 number has beign given.";
    }
}

class OneException extends Exception
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1 number has beign given.";
    }
}

class ThousandException extends Exception
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1000 number has beign given.";
    }
}

function drawNumber()
{
    $number = readline("Give some number: ");

    if ($number == 0) {
        throw new ZeroException($number);
    } elseif ($number == 1) {
        throw new OneException($number);
    } elseif ($number == 1000) {
        throw new ThousandException($number);
    }

    return $number;
}

print("Program begin...\n");

try {
    print("Risky code...\n");

    $number = drawNumber();

    print("Given number " . $number . " didn't case exception throwing.\n");
} catch (ZeroException | OneException $e) {
    print("CASE 1: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (ThousandException $e) {
    print("CASE 2: " . $e->getMessage() . " (" . $e->number . ")\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Give some number: 0
CASE 1: 0 number has beign given. (0)
Program end...

Program begin...
Risky code...
Give some number: 1
CASE 1: 1 number has beign given. (1)
Program end...

Program begin...
Risky code...
Give some number: 2
Given number 2 didn't case exception throwing.
Program end...

Program begin...
Risky code...
Give some number: 1000
CASE 2: 1000 number has beign given. (1000)
Program end...

Source code: Example

Code always executed

finally block

A finally block may also be specified after or instead of catch blocks. Code within the finally block will always be executed after the try and catch blocks, regardless of whether an exception has been thrown, and before normal execution resumes.

-- PHP Reference

Example: Exception handling with a finally block

<?php
function inverse($x) {
    if (!$x) {
        throw new Exception('Division by zero.');
    }
    return 1/$x;
}

try {
    echo inverse(5) . "\n";
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
} finally {
    echo "First finally.\n";
}

try {
    echo inverse(0) . "\n";
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
} finally {
    echo "Second finally.\n";
}

// Continue execution
echo "Hello World\n";
?>

The above example will output:

0.2
First finally.
Caught exception: Division by zero.
Second finally.
Hello World

-- PHP Reference

Example: Code always executed

<?php

try {
    $number = readline("Give some number: ");

    if ($number == 0) {
        throw new Exception('The exception has been thrown.');
    }
} catch (Exception $exception) {
    print("The exception has been catched.\n");
} finally {
    print("The code has been executed.\n");
}

Result (PHP 8.4):

Give some number: 0
The exception has been catched.
The code has been executed.

Give some number: 1
The code has been executed.

Source code: Example

One notable interaction is between the finally block and a return statement. If a return statement is encountered inside either the try or the catch blocks, the finally block will still be executed. Moreover, the return statement is evaluated when encountered, but the result will be returned after the finally block is executed. Additionally, if the finally block also contains a return statement, the value from the finally block is returned.

-- PHP Reference

Example: Interaction between the finally block and return

<?php

function test() {
    try {
        throw new Exception('foo');
    } catch (Exception $e) {
        return 'catch';
    } finally {
        return 'finally';
    }
}

echo test();
?>

The above example will output:

finally

-- PHP Reference

Another notable interaction is between an exception thrown from within a try block, and an exception thrown from within a finally block. If both blocks throw an exception, then the exception thrown from the finally block will be the one that is propagated, and the exception thrown from the try block will be used as its previous exception.

-- PHP Reference

try - catch - finally

Example: try - catch - finally

<?php

class NumberValueException extends Exception
{
    public int $number;
}

class ZeroException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "0 number has beign given.";
    }
}

class OneException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1 number has beign given.";
    }
}

class ThousandException extends NumberValueException
{
    public function __construct(int $number)
    {
        $this->number = $number;
        $this->message = "1000 number has beign given.";
    }
}

function drawNumber()
{
    $number = readline("Give some number: ");

    if ($number == 0) {
        throw new ZeroException($number);
    } elseif ($number == 1) {
        throw new OneException($number);
    } elseif ($number == 1000) {
        throw new ThousandException($number);
    } elseif ($number == 10000) {
        throw new NumberValueException();
    }
}

print("Program begin...\n");

try {
    print("Risky code...\n");

    $number = drawNumber();

    print("Given number " . $number . " didn't case exception throwing.\n");
} catch (ZeroException $e) {
    print("CASE 1: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (OneException $e) {
    print("CASE 2: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (ThousandException $e) {
    print("CASE 3: " . $e->getMessage() . " (" . $e->number . ")\n");
} finally {
    print("End of risks.\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Give some number:
Given number  didn't case exception throwing.
End of risks.
Program end...

Program begin...
Risky code...
Give some number: 0
CASE 1: 0 number has beign given. (0)
End of risks.
Program end...

Program begin...
Risky code...
Give some number: 1
CASE 2: 1 number has beign given. (1)
End of risks.
Program end...

Program begin...
Risky code...
Give some number: 2
Given number  didn't case exception throwing.
End of risks.
Program end...

Program begin...
Risky code...
Give some number: 1000
CASE 3: 1000 number has beign given. (1000)
End of risks.
Program end...

Program begin...
Risky code...
Give some number: 10000
End of risks.
PHP Fatal error:  Uncaught NumberValueException in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/try_catch_finally.php:46
Stack trace:
#0 /projects/php.lab/examples/code/errors_and_exceptions/exceptions/try_catch_finally.php(55): drawNumber()
#1 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/try_catch_finally.php on line 46

Source code: Example

Throwing and catching exception and returning value

Example: Throwing and catching an exception and returning a value

<?php

class SomeException extends Exception
{
    function __construct(public mixed $number)
    {
        $this->message = "Number has beign given.";
    }
}

class OtherException extends Exception
{
    function __construct(public mixed $value)
    {
        $this->message = "Value has beign given.";
    }
}

function someRiskySituation(): void
{
    $input = readline("Input: ");

    if (empty($input)) {
        return;
    } elseif (is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

function someFunction(): int
{
    try {
        someRiskySituation();
        return 1;
    } catch (SomeException $e) {
        print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");
        return 2;
    } catch (OtherException $e) {
        print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");
        return 3;
    } finally {
        print("Will always execute.\n");
    }

    print("Will not execute (due to return).\n");
}

$result = someFunction();
print("RETURNED: {$result}\n\n");

function otherFunction(): int
{
    try {
        someRiskySituation();
        return 1;
    } catch (SomeException $e) {
        print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");
        return 2;
    } catch (OtherException $e) {
        print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");
        return 3;
    } finally {
        print("Will always execute.\n");
        return 4;
    }

    print("Will not execute (due to return).\n");
}

$result = otherFunction();
print("RETURNED: {$result}\n\n");

Result (PHP 8.4):

Input:
Will always execute.
RETURNED: 1

Input:
Will always execute.
RETURNED: 4

Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
RETURNED: 2

Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
RETURNED: 4

Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
RETURNED: 3

Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
RETURNED: 4

Source code: Example

Nested exceptions

Example: Exception in try and in finally

<?php

try {
    try {
        throw new Exception(message: 'Third', previous: new Exception('Fourth'));
    } finally {
        throw new Exception(message: 'First', previous: new Exception('Second'));
    }
} catch (Exception $e) {
    var_dump(
        $e->getMessage(),
        $e->getPrevious()->getMessage(),
        $e->getPrevious()->getPrevious()->getMessage(),
        $e->getPrevious()->getPrevious()->getPrevious()->getMessage(),
    );
}

The above example will output:

string(5) "First"
string(6) "Second"
string(5) "Third"
string(6) "Fourth"

-- PHP Reference

Example: Nesting try-catch in try

<?php

class SomeException extends Exception
{
    function __construct(public mixed $number)
    {
        $this->message = "Number has beign given.";
    }
}

class OtherException extends Exception
{
    function __construct(public mixed $value)
    {
        $this->message = "Value has beign given.";
    }
}

function someRiskySituation()
{
    $input = readline("Input: ");

    if (empty($input)) {
        return $input;
    } elseif (is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

print("Program begin...\n");

try {
    try {
        print("Risky code...\n");

        $result = someRiskySituation();

        print("Result {$result} didn't case exception throwing.\n");
    } catch (SomeException $e) {
        print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");
    }
} catch (OtherException $e) {
    print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");
} finally {
    print("Will always execute.\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Input: 0
Result 0 didn't case exception throwing.
Will always execute.
Program end...

Program begin...
Risky code...
Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
Program end...

Program begin...
Risky code...
Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
Program end...

Source code: Example

Example: Nestring try-catch in catch

<?php

class SomeException extends Exception
{
    function __construct(public mixed $number)
    {
        $this->message = "Number has beign given.";
    }
}

class OtherException extends Exception
{
    function __construct(public mixed $value)
    {
        $this->message = "Value has beign given.";
    }
}

function someRiskySituation()
{
    $input = readline("Input: ");

    if (empty($input)) {
        return $input;
    } elseif (is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

print("Program begin...\n");

try {
    print("Risky code...\n");

    $result = someRiskySituation();

    print("Result {$result} didn't case exception throwing.\n");
} catch (SomeException $e) {
    print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");
} catch (OtherException $e) {
    print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");

    try {
        print("Another risky code...\n");

        $result = someRiskySituation();

        print("Another result {$result} didn't case exception throwing.\n");
    } catch (SomeException $e) {
        print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");
    } catch (OtherException $e) {
        print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");
    }
} finally {
    print("Will always execute.\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Input: 0
Result 0 didn't case exception throwing.
Will always execute.
Program end...

Program begin...
Risky code...
Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
Program end...

Program begin...
Risky code...
Input: a
OTHER CASE: Value has beign given. (a)
Another risky code...
Input: 0
Another result 0 didn't case exception throwing.
Will always execute.
Program end...

Program begin...
Risky code...
Input: a
OTHER CASE: Value has beign given. (a)
Another risky code...
Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
Program end...

Program begin...
Risky code...
Input: a
OTHER CASE: Value has beign given. (a)
Another risky code...
Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
Program end...

Source code: Example

Rethrowing exception

Example: Nested exception

<?php

class MyException extends Exception { }

class Test {
    public function testing() {
        try {
            try {
                throw new MyException('foo!');
            } catch (MyException $e) {
                // rethrow it
                throw $e;
            }
        } catch (Exception $e) {
            var_dump($e->getMessage());
        }
    }
}

$foo = new Test;
$foo->testing();

?>

The above example will output:

string(4) "foo!"

-- PHP Reference

Example: Rethrowing an exception in the catch block

<?php

class SomeException extends Exception
{
    function __construct(public mixed $number)
    {
        $this->message = "Number has beign given.";
    }
}

class OtherException extends Exception
{
    function __construct(public mixed $value)
    {
        $this->message = "Value has beign given.";
    }
}

function someRiskySituation(): void
{
    $input = readline("Input: ");

    if (empty($input)) {
        return;
    } elseif (is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

try {
    someRiskySituation();
} catch (SomeException $e) {
    print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");

    throw $e;
} catch (OtherException $e) {
    print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");

    throw new OtherException($e->getMessage() . ' rethrown');
} finally {
    print("Will always execute.\n");
}

Result (PHP 8.4):

Input: 0
Will always execute.

Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
PHP Fatal error:  Uncaught SomeException: Number has beign given. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch.php:26
Stack trace:
#0 /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch.php(33): someRiskySituation()
#1 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch.php on line 26

Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
PHP Fatal error:  Uncaught OtherException: Value has beign given. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch.php:41
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch.php on line 41

Source code: Example

Example: Rethrowing an exception in the catch block and throwing an exception in the finally block

<?php

class SomeException extends Exception
{
    function __construct(public mixed $number)
    {
        $this->message = "Number has beign given.";
    }
}

class OtherException extends Exception
{
    function __construct(public mixed $value)
    {
        $this->message = "Value has beign given.";
    }
}

function someRiskySituation(): void
{
    $input = readline("Input: ");

    if (empty($input)) {
        return;
    } elseif (is_numeric($input)) {
        throw new SomeException($input);
    } else {
        throw new OtherException($input);
    }
}

try {
    someRiskySituation();
} catch (SomeException $e) {
    print("SOME CASE: " . $e->getMessage() . " (" . $e->number . ")\n");

    throw $e;
} catch (OtherException $e) {
    print("OTHER CASE: " . $e->getMessage() . " (" . $e->value . ")\n");

    throw new OtherException($e->getMessage() . ' rethrown');
} finally {
    print("Will always execute.\n");

    throw new Exception('From always executed code.');
}

Result (PHP 8.4):

Input: 0
Will always execute.
PHP Fatal error:  Uncaught Exception: From always executed code. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php:45
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php on line 45

Input: 1
SOME CASE: Number has beign given. (1)
Will always execute.
PHP Fatal error:  Uncaught SomeException: Number has beign given. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php:26
Stack trace:
#0 /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php(33): someRiskySituation()
#1 {main}

Next Exception: From always executed code. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php:45
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php on line 45

Input: a
OTHER CASE: Value has beign given. (a)
Will always execute.
PHP Fatal error:  Uncaught OtherException: Value has beign given. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php:41
Stack trace:
#0 {main}

Next Exception: From always executed code. in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php:45
Stack trace:
#0 {main}
  thrown in /projects/php.lab/examples/code/errors_and_exceptions/exceptions/rethrowing_exception_in_catch_and_finally.php on line 45

Source code: Example

Global exception handler

If an exception is allowed to bubble up to the global scope, it may be caught by a global exception handler if set. The set_exception_handler() function can set a function that will be called in place of a catch block if no other block is invoked. The effect is essentially the same as if the entire program were wrapped in a try-catch block with that function as the catch.

Note:

Internal PHP functions mainly use error reporting, only modern object-oriented extensions use exceptions. However, errors can be easily translated to exceptions with ErrorException. This technique only works with non-fatal errors, however.

Example: Converting error reporting to exceptions

<?php
function exceptions_error_handler($severity, $message, $filename, $lineno) {
    throw new ErrorException($message, 0, $severity, $filename, $lineno);
}

set_error_handler('exceptions_error_handler');
?>

Tip

The Standard PHP Library (SPL) provides a good number of built-in exceptions.

Extending exceptions

A user-defined exception class can be defined by extending the built-in Exception class. The members and properties below, show what is accessible within the child class that derives from the built-in Exception class.

Example: The built-in Exception class

<?php
class Exception implements Throwable
{
    protected $message = 'Unknown exception';   // exception message
    private   $string;                          // __toString cache
    protected $code = 0;                        // user defined exception code
    protected $file;                            // source filename of exception
    protected $line;                            // source line of exception
    private   $trace;                           // backtrace
    private   $previous;                        // previous exception if nested exception

    public function __construct($message = '', $code = 0, ?Throwable $previous = null);

    final private function __clone();           // Inhibits cloning of exceptions.

    final public  function getMessage();        // message of exception
    final public  function getCode();           // code of exception
    final public  function getFile();           // source filename
    final public  function getLine();           // source line
    final public  function getTrace();          // an array of the backtrace()
    final public  function getPrevious();       // previous exception
    final public  function getTraceAsString();  // formatted string of trace

    // Overrideable
    public function __toString();               // formatted string for display
}
?>

If a class extends the built-in exception class and re-defines the constructor, it is highly recommended that it also call parent::__construct() to ensure all available data has been properly assigned. The __toString() method can be overridden to provide a custom output when the object is presented as a string.

Note:

Exceptions cannot be cloned. Attempting to clone an Exception will result in a fatal E_ERROR error.

Example: Extending the Exception class

<?php
/**
 * Define a custom exception class
 */
class MyException extends Exception
{
    // Redefine the exception so message isn't optional
    public function __construct($message, $code = 0, ?Throwable $previous = null) {
        // some code

        // make sure everything is assigned properly
        parent::__construct($message, $code, $previous);
    }

    // custom string representation of object
    public function __toString() {
        return __CLASS__ . ": [{$this->code}]: {$this->message}\n";
    }

    public function customFunction() {
        echo "A custom function for this type of exception\n";
    }
}


/**
 * Create a class to test the exception
 */
class TestException
{
    public $var;

    const THROW_NONE    = 0;
    const THROW_CUSTOM  = 1;
    const THROW_DEFAULT = 2;

    function __construct($avalue = self::THROW_NONE) {

        switch ($avalue) {
            case self::THROW_CUSTOM:
                // throw custom exception
                throw new MyException('1 is an invalid parameter', 5);
                break;

            case self::THROW_DEFAULT:
                // throw default one.
                throw new Exception('2 is not allowed as a parameter', 6);
                break;

            default:
                // No exception, object will be created.
                $this->var = $avalue;
                break;
        }
    }
}


// Example 1
try {
    $o = new TestException(TestException::THROW_CUSTOM);
} catch (MyException $e) {      // Will be caught
    echo "Caught my exception\n", $e;
    $e->customFunction();
} catch (Exception $e) {        // Skipped
    echo "Caught Default Exception\n", $e;
}

// Continue execution
var_dump($o); // Null
echo "\n\n";


// Example 2
try {
    $o = new TestException(TestException::THROW_DEFAULT);
} catch (MyException $e) {      // Doesn't match this type
    echo "Caught my exception\n", $e;
    $e->customFunction();
} catch (Exception $e) {        // Will be caught
    echo "Caught Default Exception\n", $e;
}

// Continue execution
var_dump($o); // Null
echo "\n\n";


// Example 3
try {
    $o = new TestException(TestException::THROW_CUSTOM);
} catch (Exception $e) {        // Will be caught
    echo "Default Exception caught\n", $e;
}

// Continue execution
var_dump($o); // Null
echo "\n\n";


// Example 4
try {
    $o = new TestException();
} catch (Exception $e) {        // Skipped, no exception
    echo "Default Exception caught\n", $e;
}

// Continue execution
var_dump($o); // TestException
echo "\n\n";
?>

-- PHP Reference

Exceptions hierarchy

Example: Exceptions hierarchy

<?php

class NumberValueException extends Exception
{
    function __construct(
        public int $number = 0,
        string $message = "Some number has beign given.",
        int $code = 0,
        ?Throwable $previous = null,
    ) {
        parent::__construct($message, $code, $previous);
    }

    public function __toString() {
        return __CLASS__ . ": [{$this->code}]: {$this->message} ($this->number)";
    }
}

class ZeroException extends NumberValueException
{
    function __construct(
        public int $number = 0,
        string $message = "0 number has beign given.",
        int $code = 0,
        ?Throwable $previous = null,
    ) {
        parent::__construct($number, $message, $code, $previous);
    }
}

class OneException extends NumberValueException
{
    function __construct(
        public int $number = 1,
        string $message = "1 number has beign given.",
        int $code = 0,
        ?Throwable $previous = null,
    ) {
        parent::__construct($number, $message, $code, $previous);
    }
}

class ThousandException extends NumberValueException
{
    function __construct(
        public int $number = 1000,
        string $message = "1000 number has beign given.",
        int $code = 0,
        ?Throwable $previous = null,
    ) {
        parent::__construct($number, $message, $code, $previous);
    }
}

function draw_number(): int
{
    $number = readline("Give some number: ");

    if ($number == 0) {
        throw new ZeroException($number);
    } elseif ($number == 1) {
        throw new OneException($number);
    } elseif ($number == 10) {
        throw new NumberValueException($number);
    } elseif ($number == 1000) {
        throw new ThousandException($number);
    }

    return $number;
}

print("Program begin...\n");

try {
    print("Risky code...\n");

    $number = draw_number();

    print("Given number {$number} didn't case exception throwing.\n");
} catch (ZeroException $e) {
    print("CASE 1: {$e->getMessage()} ({$e->number})\n{$e}\n");
} catch (OneException $e) {
    print("CASE 2: {$e->getMessage()} ({$e->number})\n{$e}\n");
} catch (NumberValueException $e) {
    print("CASE 0: {$e->getMessage()} ({$e->number})\n{$e}\n");
} catch (ThousandException $e) { // Will be never catched.
    print("CASE 3: {$e->getMessage()} ({$e->number})\n{$e}\n");
}

print("Program end...\n");

Result (PHP 8.4):

Program begin...
Risky code...
Give some number: 0
CASE 1: 0 number has beign given. (0)
NumberValueException: [0]: 0 number has beign given. (0)
Program end...

Program begin...
Risky code...
Give some number: 1
CASE 2: 1 number has beign given. (1)
NumberValueException: [0]: 1 number has beign given. (1)
Program end...

Program begin...
Risky code...
Give some number: 2
Given number 2 didn't case exception throwing.
Program end...

Program begin...
Risky code...
Give some number: 10
CASE 0: Some number has beign given. (10)
NumberValueException: [0]: Some number has beign given. (10)
Program end...

Program begin...
Risky code...
Give some number: 1000
CASE 0: 1000 number has beign given. (1000)
NumberValueException: [0]: 1000 number has beign given. (1000)
Program end...

Source code: Example

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