PHP Predefined Variables

PHP provides several predefined variables that are always available to a script. These variables are often referred to as superglobals, because they are available in all scopes throughout a script. Here are some of the most commonly used ones:

$_GET: An associative array of variables passed to the current script via the URL parameters.

// URL: www.example.com/test.php?name=John
echo $_GET["name"]; // Outputs: John

$_POST: An associative array of variables passed to the current script via the HTTP POST method.

// Assuming a form submitted "name" as "John"
echo $_POST["name"]; // Outputs: John

$_FILES: An associative array of items uploaded to the current script via the HTTP POST method.

echo $_FILES['userfile']['name']; // Outputs the name of the uploaded file

$_COOKIE: An associative array of variables passed to the current script via HTTP cookies.

echo $_COOKIE["user"]; // Outputs the value of the cookie named "user"

$_SESSION: An associative array containing session variables available to the current script.

echo $_SESSION["username"]; // Outputs the value of the session variable "username"

$_SERVER: An associative array containing information such as headers, paths, and script locations.

echo $_SERVER['HTTP_HOST']; // Outputs the Host header from the current request

$_REQUEST: An associative array that by default contains the contents of $_GET, $_POST and $_COOKIE.

echo $_REQUEST["name"]; // Outputs the value of "name" from either $_POST, $_GET or $_COOKIE

$_ENV: An associative array of variables passed to the current script via the environment method.

echo $_ENV["PATH"]; // Outputs the value of the environment variable "PATH"

$GLOBALS: An associative array containing references to all variables which are currently defined in the global scope of the script.

$x = 75;
$y = 25;

function addition() {
    $GLOBALS['z'] = $GLOBALS['x'] + $GLOBALS['y'];
}

addition();
echo $z; // Outputs: 100

Remember that these predefined variables are always available, regardless of the scope of your script.

$_GET, $_POST, and $_REQUEST in PHP:
- $_GET is used to collect form data after submitting an HTML form with the method="get" attribute. $_POST is used with method="post". $_REQUEST contains data from both $_GET and $_POST.
```
// Example using $_GET
$username = $_GET['username'];

// Example using $_POST
$password = $_POST['password'];

// Example using $_REQUEST
$email = $_REQUEST['email'];
```

Working with $_SESSION and $_COOKIE variables in PHP:

$_SESSION is used to store session variables, and $_COOKIE holds cookie variables.

// Using $_SESSION
session_start();
$_SESSION['user_id'] = 123;

// Using $_COOKIE
setcookie('user_language', 'en', time() + (86400 * 30), "/");

Server-related variables like $_SERVER in PHP:
- $_SERVER contains information about the server and the execution environment.
```
$serverName = $_SERVER['SERVER_NAME'];
$userAgent = $_SERVER['HTTP_USER_AGENT'];
```

Using $_FILES for file upload handling in PHP:

$_FILES is used to access information about uploaded files.

$uploadedFile = $_FILES['file'];
$fileName = $uploadedFile['name'];
$fileTemp = $uploadedFile['tmp_name'];

Accessing environment variables with $_ENV in PHP:
- $_ENV holds environment variables.
```
$databaseUser = $_ENV['DB_USER'];
$databasePassword = $_ENV['DB_PASSWORD'];
```
PHP magic constants and their role as predefined variables:
- Magic constants are predefined constants in PHP. Some include __FILE__, __LINE__, __DIR__, __FUNCTION__, __CLASS__, and __NAMESPACE__.
```
echo "This script is in: " . __FILE__;
echo "This function is: " . __FUNCTION__;
```