PHP Global Variables: Superglobals Explained
Understanding PHP Superglobals
Superglobals in PHP are predefined variables that are always accessible, regardless of the scope.
What Are PHP Superglobals?
Superglobals in PHP are special arrays that provide access to variables from various scopes throughout a script.
TLDR; Quick Overview of PHP Superglobals
There are several superglobals in PHP which include $_GET, $_POST, $_REQUEST, $_SESSION, $_COOKIE, $_SERVER, $_ENV, $_FILES, and $_GLOBALS.
They are used to retrieve data from forms, manage user sessions, keep track of server information, and more.
PHP version 4.1.0 was instrumental in introducing these elements, so they work best with PHP versions 4.1.0 and up.
Deeper Dive into PHP Superglobals
Let’s unpack these arrays one by one offering examples.
$_GET is an array used primarily to collect data submitted in HTML form using the GET method.
Here is a classic example:
<form action="test_get.php" method="get">
Name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>
<?php
echo "Hi " . $_GET['fname'];
?>
$_POST behaves similarly to $_GET, but it handles data that is sent via HTTP POST.
This is often used for form submissions with sensitive information, like passwords, because it does not display the data in the URL:
<form action="test_post.php" method="post">
Password: <input type="password" name="password">
<input type="submit" value="Submit">
</form>
<?php
echo "Your password is " . $_POST['password'];
?>
$_REQUEST, on the other hand, can collect data from both GET and POST methods.
For tracking website sessions, $_SESSION is your go-to global.
Setting up a user session might look something like this:
<?php
session_start();
$_SESSION["favcolor"] = "green";
$_SESSION["favanimal"] = "cat";
echo "Session variables are set.";
?>
Cookies are managed through $_COOKIE.
Let’s say you want to store a user’s name in a cookie; it might look like:
<?php
$cookie_name = "user";
$cookie_value = "John Doe";
setcookie($cookie_name, $cookie_value, time() + (86400 * 30), "/"); // 86400 = 1 day
?>
<?php
if(!isset($_COOKIE[$cookie_name])) {
echo "Cookie named '" . $cookie_name . "' is not set!";
} else {
echo "Cookie '" . $cookie_name . "' is set!
";
echo "Value is: " . $_COOKIE[$cookie_name];
}
?>
$_SERVER is a vast array containing information such as headers, paths, and script locations.
A frequently accessed $_SERVER variable is $_SERVER['HTTP_USER_AGENT'], which gives you user’s browser details.
And $_ENV is an array of variables passed to the current script via the environment method.
For file uploads, there is a special variable $_FILES.
A simple file upload form example:
<form action="upload.php" method="post" enctype="multipart/form-data">
Select image to upload:<br>
<input type="file" name="fileToUpload" id="fileToUpload"><br>
<input type="submit" value="Upload Image" name="submit">
</form>
Lastly, $_GLOBALS is a PHP superglobal variable which is used to access global variables from anywhere in the PHP script.
Best Practices and Security Concerns
PHP superglobals make web development simpler, but precautionary measures are essential for security.
Always validate and sanitize user inputs when using superglobals to prevent security vulnerabilities such as cross-site scripting (XSS) and SQL injection.
For instance, use the htmlspecialchars() function to sanitize data from $_GET or $_POST before outputting it to browser.
When dealing with databases, prepared statements and parameterized queries should be used to prevent SQL injection.
Furthermore, be aware that register_globals is deprecated as it can lead to security issues.
Only start a session after a proper user login procedure to avoid session hijacking.
And always set secure flags for cookies if your application uses https.
Common Issues and Solutions with Superglobals
One common mistake is failing to start the session with session_start() before using $_SESSION.
Another issue is relying on $_REQUEST which contains both GET, POST, and COOKIE data, and might introduce security concerns.
Sometimes, file upload errors occur due to server configurations; always check the php.ini settings for upload_max_filesize and post_max_size.
Frequently Asked Questions
Why are superglobals called “superglobals” in PHP?
They are accessible from anywhere in the script, regardless of the scope – hence “super”.
Can superglobals be disabled for security?
No, they are a core part of PHP and cannot be disabled, but you should practice safe input handling.
Is register_globals related to PHP superglobals?
It was a PHP directive allowing for automatic globals creation. This is deprecated due to security issues.
How can I prevent users from seeing sensitive data passed through $_GET?
Use $_POST for sensitive data, as it does not display data in the URL.
Are PHP superglobals available in all versions of PHP?
They are available from PHP version 4.1.0 onwards.
Can I use $_FILES for multiple file uploads?
Yes, set the file input name attribute to an array (e.g., name=”files[]”) and iterate over $_FILES accordingly.
Understanding the Scope and Accessibility of Superglobals
Despite being accessible globally, the scope of a superglobal is not to be mistaken for a global variable that you declare yourself.
Unlike regular global variables, superglobals are built into the PHP language itself, which means they are present and ready to use without any additional declaration.
Security Measures for Using Superglobals in Forms
While retrieving information from forms, it is crucial to employ security measures to keep your application safe.
To enhance form handling security, employ CSRF tokens, and validate all user inputs using the back-end script, regardless of client-side validation.
Utilizing Superglobals for Session Management
$_SESSION superglobals are crucial for maintaining data across your application pages.
Remember to regenerate session IDs after login to enhance session security and thwart session fixation attacks.
The Importance of Proper Environment Variables in $_ENV
With $_ENV, you can retrieve environment variables which is useful for reading configuration settings without hard-coding them into your application.
However, remember to keep sensitive information such as database passwords out of document roots and server document indexes to mitigate risks.
Handling File Uploads Safely with $_FILES
When using $_FILES, always check for the uploaded file’s MIME types and ensure proper validation to prevent malicious file uploads.
Additionally, control the size and type of files allowed for upload to avoid any unnecessary server overload or security breach.
Accessing Global Variables Anywhere with $_GLOBALS
$_GLOBALS serves as the gateway to access all global variables anywhere in your PHP script.
Also, it’s crucial to note that while $_GLOBALS is available to access global variables created by the user, moderation in its use is advised to maintain clear code.
Security Considerations When Using $_COOKIE
It’s essential when employing $_COOKIE, to secure it with HTTPOnly flags to protect against cross-site scripting (XSS) attacks.
For cookies that are critical for session management, such as session cookies, utilizing attributes like Secure, SameSite, and HTTPOnly can significantly enhance security.
Retrieving Server Information via $_SERVER
Accessing data such as script locations, user IP addresses, and other server metadata through $_SERVER is straightforward but ensure to filter and sanitize any data that you output to the user to prevent information leakage.
For example, be cautious when revealing server paths or IP addresses as they can be leveraged by malicious entities.
The Evolution of PHP Superglobals
PHP superglobals came into play with the release of PHP 4.1.0, marking a significant evolution in the way PHP handles variable accessibility.
Their implementation has been crucial for ease of use while maintaining proper security practices.
Proper Use of Superglobals in AJAX Requests
Using superglobals in conjunction with AJAX allows for seamless dynamic content loading without the need for page refreshes.
Remember to treat all data from AJAX requests with the same security considerations as any other user input.
Understanding the Difference Between $_GET and $_POST
The choice between $_GET and $_POST often depends on the kind of data that is being transmitted and the action being performed by the script.
$_GET is generally used for non-sensitive data retrieval or when bookmarking a specific URL with parameters is desired, while $_POST is used for transmitting data that should not be visible or should not alter the server state when resubmitting a form.
Frequently Asked Questions
Do superglobals sanitize input data for me?
No, PHP superglobals do not sanitize input; it is the developer’s responsibility to perform input validation and sanitization.
How can I ensure $_FILES handles uploads securely?
Implement server-side file validation, limit the file types and sizes, and store uploaded files outside the web root when possible.
Are $_COOKIE variables safe to store sensitive information?
No, it is not safe to store sensitive information in cookies. Use sessions and secure, encrypted storage for sensitive data.
Can the values in $_SERVER be altered by the user or external sources?
Some values in $_SERVER can be influenced by the user, like the HTTP headers. Always treat them as untrusted input.
What are some best practices when using $_SESSION to handle user data?
Always start sessions with session_start(), use HTTPS, regenerate session IDs post-login, and use secure and HTTPOnly cookies.
