PHP Global Variables Tutorial with Examples | Superglobals in PHP

Table of Contents:

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In this detailed course page, we’ll explore PHP Global Variables, which are a fundamental concept in PHP programming. Understanding global variables is essential because they allow data to be accessed across multiple scopes of your script.

By the end of this tutorial, you will:

Understand the difference between local, global, and static variables
Learn how to use the $GLOBALS superglobal array
Work with PHP Superglobals ($_POST, $_GET, $_REQUEST, $_SERVER, $_SESSION, $_COOKIE, $_FILES, $_ENV)
Discover best practices for handling global variables securely
Build real-world examples like forms, sessions, and cookie management

You will Learn

What are Global Variables in PHP?
Variable Scope in PHP
- Local Scope
- Global Scope
- Static Scope
The $GLOBALS Superglobal
Examples of Using $GLOBALS
PHP Superglobals Overview
$_SERVER Superglobal
$_REQUEST Superglobal
$_POST Superglobal
$_GET Superglobal
$_SESSION Superglobal
$_COOKIE Superglobal
$_FILES Superglobal
$_ENV Superglobal
Difference Between $_POST and $_GET
Using Global Variables in Functions
Security Concerns with Global Variables
Real-World Example: Login System with $_POST and $_SESSION
Real-World Example: Contact Form with $_POST
Real-World Example: Tracking Visitors with $_SERVER
Best Practices for Using Global Variables
Common Mistakes with Global Variables
Advanced Usage with Superglobals
PHP Global Constants vs. Global Variables
Functions vs. Global Variables
Debugging Global Variables
How Global Variables Work in Large Applications
Organizing Superglobal Usage in PHP Projects
PHP Global Variables and Object-Oriented Programming

1. What are Global Variables in PHP?

In PHP, a global variable is a variable declared outside of a function and accessible from any part of the script (with the global keyword or $GLOBALS).

Example:

$x = 50;

function test() {
    global $x;
    echo $x; // Accessible inside function using global
}

test(); // 50

2. Variable Scope in PHP

PHP has three main types of variable scope:

Local Scope – Variable declared inside a function.
Global Scope – Variable declared outside functions.
Static Scope – Variables that retain value after function execution.

Local Scope Example

function localScope() {
    $x = 10; 
    echo $x;
}
localScope(); // 10
echo $x; // Error: undefined

Global Scope Example

$y = 100;

function globalScope() {
    global $y;
    echo $y;
}
globalScope(); // 100

Static Scope Example

function counter() {
    static $count = 0;
    $count++;
    echo $count;
}
counter(); // 1
counter(); // 2
counter(); // 3

3. The $GLOBALS Superglobal

The $GLOBALS array is an associative array containing all global variables.

$a = 5;
$b = 10;

function add() {
    $GLOBALS['c'] = $GLOBALS['a'] + $GLOBALS['b'];
}

add();
echo $c; // 15

4. Examples of Using $GLOBALS

Sharing data between functions
Accessing configuration variables
Avoiding duplication of code

5. PHP Superglobals Overview

Superglobals are built-in global arrays available in all scopes:

$GLOBALS
$_SERVER
$_REQUEST
$_POST
$_GET
$_SESSION
$_COOKIE
$_FILES
$_ENV

6. $_SERVER Superglobal

$_SERVER contains server and execution environment information.

echo $_SERVER['PHP_SELF'];  
echo $_SERVER['SERVER_NAME'];  
echo $_SERVER['HTTP_USER_AGENT'];

7. $_REQUEST Superglobal

$_REQUEST is used to collect form data sent by GET or POST.

$name = $_REQUEST['username'];
echo "Welcome $name";

8. $_POST Superglobal

$_POST is used for form submissions (secure).

<form method="post">
  Name: <input type="text" name="user">
  <input type="submit">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    echo "Hello " . $_POST['user'];
}
?>

9. $_GET Superglobal

$_GET is used for URL parameters.

<a href="page.php?name=John">Click</a>

<?php
echo $_GET['name']; // John
?>

10. $_SESSION Superglobal

$_SESSION is used to store data across multiple pages.

session_start();
$_SESSION['username'] = "Alice";
echo $_SESSION['username'];

11. $_COOKIE Superglobal

$_COOKIE stores small data on the client browser.

setcookie("user", "John", time()+3600);

if(isset($_COOKIE['user'])) {
    echo "Welcome " . $_COOKIE['user'];
}

12. $_FILES Superglobal

Handles file uploads.

<form method="post" enctype="multipart/form-data">
  <input type="file" name="fileUpload">
  <input type="submit">
</form>

move_uploaded_file($_FILES["fileUpload"]["tmp_name"], "uploads/" . $_FILES["fileUpload"]["name"]);

13. $_ENV Superglobal

Contains environment variables like OS paths.

print_r($_ENV);

14. Difference Between $_POST and $_GET

$_POST – Secure, does not show data in URL, used for forms.
$_GET – Data visible in URL, used for links and bookmarks.

15. Using Global Variables in Functions

$greeting = "Hello";

function display() {
    global $greeting;
    echo $greeting;
}
display(); // Hello

16. Security Concerns with Global Variables

Avoid exposing sensitive data via $_GET
Sanitize $_REQUEST inputs
Use sessions instead of cookies for critical data

17. Real-World Example: Login System with $_POST and $_SESSION

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $user = $_POST['username'];
    $pass = $_POST['password'];
    
    if ($user == "admin" && $pass == "1234") {
        session_start();
        $_SESSION['user'] = $user;
        echo "Login successful!";
    } else {
        echo "Invalid credentials";
    }
}

18. Real-World Example: Contact Form with $_POST

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = $_POST['name'];
    $message = $_POST['message'];
    echo "Thank you $name. Your message: $message";
}

19. Real-World Example: Tracking Visitors with $_SERVER

$ip = $_SERVER['REMOTE_ADDR'];
echo "Your IP address is: " . $ip;

20. Best Practices for Using Global Variables

Minimize usage of global variables
Use functions and OOP instead
Sanitize all user input before processing

21. Common Mistakes with Global Variables

Overusing global keyword
Directly trusting $_GET or $_POST
Forgetting to session_start() before using sessions

22. Advanced Usage with Superglobals

Combining $_GET and $_POST with $_REQUEST
Using $_SERVER for security checks
File uploads with $_FILES

23. PHP Global Constants vs. Global Variables

Constants – Value cannot change (define("PI", 3.14))
Variables – Value can be updated

24. Functions vs. Global Variables

Functions provide encapsulation
Global variables provide shared state

25. Debugging Global Variables

print_r($GLOBALS);

26. How Global Variables Work in Large Applications

Use config files for storing global settings
Use dependency injection instead of excessive globals

27. Organizing Superglobal Usage in PHP Projects

Centralize handling in a Request class
Validate data in input filters

28. PHP Global Variables and OOP

Instead of globals:

class User {
    public $name;
    function setName($name) {
        $this->name = $name;
    }
}

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Frequently Asked Questions (FAQ)

Q1: What are global variables in PHP?
A: Global variables are accessible from anywhere in the script using global or $GLOBALS.

Q2: What are PHP Superglobals?
A: Predefined arrays like $_POST, $_GET, $_SESSION, available globally.

Q3: When should I use $_POST vs $_GET?
A: Use $_POST for secure data (forms), $_GET for URL parameters.

Q4: Are global variables safe?
A: No, they can lead to security issues if not handled properly.

Q5: What is the difference between $GLOBALS and global keyword?
A: $GLOBALS is an array; global imports the variable into function scope.