edit.php

<?php
include "db.php";
$id = $_GET['id'];
$data = $conn->query("SELECT * FROM students WHERE id=$id")->fetch_assoc();

if ($_SERVER['REQUEST_METHOD'] == "POST") {
    $name  = $_POST['name'];
    $class = $_POST['class'];
    $phone = $_POST['phone'];

    $sql = "UPDATE students SET name='$name', class='$class', phone='$phone' WHERE id=$id";
    $conn->query($sql);

    header("Location: index.php");
}
?>

<h2>Edit Student</h2>
<form method="post">
    Name: <input type="text" name="name" value="<?= $data['name'] ?>"><br><br>
    Class: <input type="text" name="class" value="<?= $data['class'] ?>"><br><br>
    Phone: <input type="text" name="phone" value="<?= $data['phone'] ?>"><br><br>
    <button type="submit">Update</button>
</form>