Introducing PHP Superglobals

Superglobals are specially-defined array variables in PHP that make it easy for you to get information about a request or its context. They are called superglobal because they are always accessible, regardless of the scope — that is, you can access them from any function, class or file without having to do anything special. The superglobal variables are: $GLOBALS, $_SERVER, $_GET, $_POST, $_FILES, $_COOKIE, $_SESSION, $_REQUEST and $_ENV. And while you can learn more about each by reading the PHP documentation, I’d like to show you the ones I think you’re likely use the most.

$_GET and $_POST

$_GET and $_POST are used to get information sent in the page request. Parameters sent in the URL are made available by $_GET. For example, this URL:

http://example.com/profile.php?uid=12345

… will send the parameter uid to profile.php, which you can then access the value in profile.php as $_GET["uid"].

When you have a form submission that is sent using method="get", the values are appended to the URL so they are available using $_GET as well. Forms submitted using method="post" send their values in the body of the HTTP request; posted data is available in your script by using $_POST. Here’s a sample web form which posts a username and password to a login script:

<form action="login.php" method="post">
Username: <input type="text" name="uname"><br>
Password: <input type="password" name="pass"><br>
<input type="submit" value="Login">
</form>

The uname value and pass value will be available as $_POST["uname"] and $_POST["pass"] respectively in login.php.

The $_REQUEST superglobal is an amalgam of $_POST and $_GET data. It’s offered as a convenience, but as a general rule I advice people to avoid it. It creates some of the same security risks that registered globals did years ago if you’re not careful. (If you don’t know what I mean by registered globals, consider yourself lucky. Just forget about $_REQUEST and you’ll be fine.)

$_SESSION

The HTTP protocol, the mechanism by which browsers request pages and receive them, was originally designed so that each request was independent from all the others made to the server. This isn’t how we use the web today, though. Now people expect each page they visit in a site to “remember” what happened previously, whether it’s logging in or adding a new video game to their online shopping cart. This logical link between pages is called a session.

To enable sessions, the session_start() function must be called at the top of any PHP script using sessions before output is sent to the browser. Once the session is active, you can store and retrieve data using $_SESSION. Here’s an example…

page1.php starts the session, stores a value in the $_SESSION array and then provides a link to page2.php:

<?php
session_start();
$_SESSION["videogame"] = "Battletoads";
?>
<a href="page2.php">Go to page 2</a>

When the user clicks on the link to page2.php, it calls session_start() to resume the session, retrieves the value from $_SESSION["videogame"] and displays it:

<?php
session_start();
echo $_SESSION["videogame"];

$_SERVER

The $_SERVER superglobal array stores information about both the web server and the request made to it. Some of the more useful $_SERVER values are:

• $_SERVER["PHP_SELF"] – the name of the currently running PHP script
• $_SERVER["REQUEST_METHOD"] – which HTTP method was used to request the page (GET, POST, etc)
• $_SERVER["REQUEST_TIME"] – the time the page request was received, represented as an integer known as a unix timestamp
• $_SERVER["HTTP_REFERER"] – the address of the previously visited page, if available

A list of all the available keys can be found in the online documentation. Keep in mind that not every one of them listed is guaranteed to be in $_SERVER; it depends on your server configuration and the request sent by the user.

Summary

As I said, these are just a few of the superglobals available in PHP which I think you’ll use the most. $_GET and $_POST allow your script to take in information from the outside world. $_SESSION lets you remember data between page requests during a visitor’s browsing session. $_SERVER holds information about the web server and the page request. Understanding these and learning to use them effectively will no doubt help you write great, interactive websites using PHP with relative ease.

Image via Tom Wang / Shutterstock