Solving the Call to Undefined Function session_register() Error: A Comprehensive Guide

Are you getting an error in your PHP application that says "Call to Undefined Function session_register()"? Don't worry! This comprehensive guide will help you understand the cause of this error and provide step-by-step solutions to fix it. We'll also answer some frequently asked questions related to this error.

Table of Contents

  1. Understanding the Error
  2. Fixing the Error
  3. Step 1: Replace session_register()
  4. Step 2: Replace session_is_registered()
  5. Step 3: Replace session_unregister()
  6. FAQs

Understanding the Error

The error "Call to Undefined Function session_register()" occurs because session_register() has been removed in PHP 5.4.0 and later versions. The same applies to session_is_registered() and session_unregister() functions. These functions have been deprecated in favor of using the $_SESSION superglobal to manage session variables.

This means that if you're using PHP 5.4.0 or later, you'll encounter this error when trying to use any of these deprecated functions. To fix this error, you'll need to update your code to use the $_SESSION superglobal instead.

Fixing the Error

Follow these steps to replace the deprecated functions with the $_SESSION superglobal in your code:

Step 1: Replace session_register()

Find all instances of session_register() in your code and replace them with the appropriate assignment to the $_SESSION superglobal. For example, if you have:


Replace it with:

$_SESSION["user"] = true;

Step 2: Replace session_is_registered()

Find all instances of session_is_registered() in your code and replace them with the isset() function to check if a session variable is set. For example, if you have:

if (session_is_registered("user")) {
    // ...

Replace it with:

if (isset($_SESSION["user"])) {
    // ...

Step 3: Replace session_unregister()

Find all instances of session_unregister() in your code and replace them with the unset() function to remove a session variable. For example, if you have:


Replace it with:



1. What is the $_SESSION superglobal?

The $_SESSION superglobal is an array that stores session variables. These variables are available across multiple pages and are used to preserve data across subsequent accesses by the user. To learn more about sessions and the $_SESSION superglobal, visit the official PHP documentation on sessions.

2. What are the advantages of using the $_SESSION superglobal over the deprecated functions?

The $_SESSION superglobal provides a more direct and efficient way of managing session variables. It is also compatible with newer versions of PHP, ensuring that your code remains up-to-date and functional.

3. How do I start a session in PHP?

To start a session in PHP, use the session_start() function at the beginning of your script, before any output is sent to the browser. For example:

// Your code here

4. How do I destroy a session in PHP?

To destroy a session and remove all session variables, use the session_destroy() function. For example:


5. How can I check if a session is active in PHP?

In PHP 5.4.0 and later versions, you can use the session_status() function to check if a session is active. The function returns one of the following constants:

  • PHP_SESSION_DISABLED: Sessions are disabled.
  • PHP_SESSION_NONE: Sessions are enabled, but no session exists.
  • PHP_SESSION_ACTIVE: Sessions are enabled, and a session exists.

For example:

if (session_status() == PHP_SESSION_NONE) {

This code checks if a session is active, and if not, it starts a new session.

Related: How to Check If a Session Variable Is Set in PHP


By following the steps provided in this guide, you should now have successfully resolved the "Call to Undefined Function session_register()" error in your PHP application. Remember to always use the $_SESSION superglobal to manage session variables and keep your code compatible with newer versions of PHP. If you have any further questions or need more help, refer to the official PHP documentation or consult the PHP community for assistance.

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