PHP

As Interpreter

Runs the written php file in terminal

php script.php

As Runtime

php files served by web server such as apache or nginx in directories like htdocs or /var/www/html are interpreted by PHP runtime extension called by the webserver

Superglobals

$_POST

Access to data sent to server from client through HTTP POST method

<?php
if (isset($_POST['data'])) {
    echo $_POST['data'];
}

$_GET

Access to data sent to server from client through HTTP GET method

<?php
if (isset($_GET['data'])) {
    echo $_GET['data'];
}

$_REQUEST

Access to data sent to server from client through any HTTP method

if (isset($_REQUEST['data'])) {
    echo $_REQUEST['data'];
}

Example Form

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Number Form</title>
</head>
<body>
    <form action="power.php" method="POST">
        <label for="number">Enter a number:</label>
        <input type="number" name="number" id="number" required>
        <button type="submit">Submit</button>
    </form>
</body>
</html>

<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['number'])) {
    $number = (int) $_POST['number'];
    $result = $number ** 2;
        
    echo "The square of $number is: $result";
} else {
    echo "Invalid request.";
}
?>

Headers