Fixing the 'Warning: Class __php_incomplete_class Has No Unserializer' Issue: A Comprehensive Guide

Learn how to resolve the __php_incomplete_class error in PHP with this comprehensive guide. We'll walk you through the causes of this issue and provide step-by-step instructions to fix it.

Table of Contents

Understanding the __php_incomplete_class Error

The __php_incomplete_class error occurs when you try to unserialize an object whose class definition is not yet loaded, causing PHP to generate a placeholder object of type __php_incomplete_class. This is typically caused by using unserialize() before including the required class definition file.

Here's an example of how this issue might arise:

// MyClass.php
class MyClass {
    public $property;

// index.php
$obj = new MyClass();
$_SESSION['obj'] = serialize($obj);

// another-file.php
$obj = unserialize($_SESSION['obj']); // Throws the warning

In the example above, the MyClass definition is not available in another-file.php, leading to the __php_incomplete_class warning.

Step-by-Step Solution to Fix the Issue

To fix the __php_incomplete_class issue, follow these steps:

Identify the missing class definition: Determine which class is not loaded when the unserialize() function is called.

Include the class definition: Make sure the class definition is included before calling unserialize(). This can be done using require, require_once, include, or include_once. It is recommended to use require_once to avoid duplicate inclusion.

// another-file.php
require_once 'MyClass.php';
$obj = unserialize($_SESSION['obj']);

Use an autoloader: If you're using multiple classes, it's a good idea to use an autoloader to automatically include class definitions when needed. You can use the built-in spl_autoload_register() function or use an external library like Composer.

// autoload.php
spl_autoload_register(function ($class_name) {
    require_once $class_name . '.php';

// another-file.php
require_once 'autoload.php';
$obj = unserialize($_SESSION['obj']);


1. What is the __php_incomplete_class error?

The __php_incomplete_class error occurs when you try to unserialize an object in PHP without having its class definition loaded. This results in a placeholder object of type __php_incomplete_class.

2. How do I fix the __php_incomplete_class error?

To fix the __php_incomplete_class error, make sure to include the class definition before calling unserialize(). You can also use an autoloader to automatically include class definitions when needed.

3. What is the difference between require and include in PHP?

Both require and include are used to include a file in another PHP script. The main difference is that require will produce a fatal error and halt script execution if the file is not found, while include will only produce a warning and continue executing the script.

4. What is the purpose of an autoloader in PHP?

An autoloader in PHP is used to automatically load class definitions when they are needed. This eliminates the need to manually include class definition files in your scripts, making your code more organized and easier to maintain.

5. How can I use Composer for autoloading in PHP?

To use Composer for autoloading in PHP, simply follow these steps:

Install Composer.

Create a composer.json file in your project root directory.

Add an autoload section to the composer.json file with the appropriate configuration for your project. For example:

  "autoload": {
    "psr-4": {
      "MyNamespace\\": "src/"

Run composer install to generate the vendor/autoload.php file.

Include the vendor/autoload.php file in your scripts to enable Composer's autoloading.

For more information on using Composer for autoloading, refer to the official documentation.

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