How do I fix 'Autoload error' after installing packages with Composer?

When encountering Composer's "Autoload error", the first thing to do is to clarify the core of the problem: PHP cannot find the required class through automatic loading. The following are the solutions: 1. Run composer dump-autoload to regenerate the automatic loading file, and clear the cache if necessary; 2. Check whether the case of the class name and file path match, especially on case-sensitive systems; 3. Check the PSR-4 automatic loading configuration in composer.json to ensure that the namespace and directory path are correct; 4. Try to uninstall and reinstall the problem package or clean the vendor directory and then reinstall it; 5. Troubleshoot duplicate class names or conflicting files. In most cases, after these problems are troubleshooted, the automatic loading error can be resolved.

You're working with Composer, install a package, and suddenly you see an "Autoload error" . It's not always the same exact message, but it usually means PHP can't find a class it needs after requiring the autoloader. The good news is, most of these issues are easy to fix once you know what to check.

1. Run composer dump-autoload

This is the most common fix. When you install or update packages, Composer generates an autoload file that tells PHP where to find classes. If something went wrong during installation or if files changed, this file might be out of date.

Try running:

 composer dump-autoload

Sometimes even after installing a package, the autoload file isn't updated properly. This command regenerates it and often clears up the issue.

If that doesn't help, try:

• Clearing the cache: composer clear-cache
• Then dumping again

2. Check for Case Sensitivity in Class Names and File Paths

PHP doesn't care about case in function or class names, but your filesystem might. If your server runs on a case-sensitive OS (like Linux), then MyClass.php and myclass.php aren't the same.

Make sure:

• Class names in files match exactly what's referenced
• Autoload rules in composer.json use the correct namespace/case

For example, if your namespace is App\Utils , but your folder is named utils , PHP won't find it on a case-sensitive system.

3. Verify Your composer.json Autoload Settings

If you're using custom namespaces or PSR-4 autoload rules, make sure they're set up correctly. A typo or incorrect path will break everything.

Here's a basic example of a proper setup:

 "autoload": {
    "psr-4": {
        "App\\": "src/"
    }
}

Things to double-check:

• Backslashes ( \ ) in namespace must be escaped with another backslash ( \\ )
• Directory paths are relative to the project root
• After editing composer.json , always run dump-autoload

4. Try Reinstalling the Problematic Package

Sometimes the downloaded files may be incomplete or corrupted, especially if there was a connection issue during install.

You can remove and reinstall like this:

 composer remove vendor/package
composer requires vendor/package

This ensures all files are fresh and properly registered.

Also, if multiple packages are involved, it might help to delete the vendor/ directory and composer.lock , then run composer install again.

5. Watch Out for Conflicting Class Names or Duplicate Files

If two different packages define the same class name, or if you have a manually created class that overlas with one from a package, PHP will get confused.

Check:

• If the error points to a specific class, search your project for duplicates
• Make sure no local files are overriding vendor ones by accident

Composer doesn't handle class conflicts well, so keeping your namespace clean helps avoid weird autoload errors.

Basically that's it.
Most autoload issues come down to outdated autoload files, incorrect config, or file system mismatches. Start with dump-autoload , check your composer settings, and don't overlook case sensitivity or duplicate files. Once those are ruled out, things usually start working again.

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