PHP Fatal error: Class "X" not found

1. About the error
2. Why do I see this error
3. Solution
4. Check the namespace matches the path
5. Import the class
6. Regenerate the autoloader

#About the error

PHP Fatal error: Uncaught Error: Class "App\Services\Invoice" not found

PHP tried to use a class and couldn't find a definition for it. With Composer's PSR-4 autoloading, the class name and namespace must map exactly to a file on disk, and any mismatch breaks that lookup.

#Why do I see this error

• The namespace declared in the file doesn't match its folder.
• The file name doesn't match the class name (PSR-4 is case-sensitive, and so is Linux).
• You forgot a use statement, so PHP looks for the class in the current namespace.
• The class is new and the autoloader hasn't been regenerated.
• A typo in the class name.

#Solution

#Check the namespace matches the path

Under PSR-4, App\Services\Invoice must live at app/Services/Invoice.php and declare:

namespace App\Services;

class Invoice
{
    // ...
}

The file name (Invoice.php) must match the class name exactly, including case. This works on macOS (case-insensitive filesystem) and then breaks on a Linux server, a very common "works locally, 500 in production" trap.

#Import the class

If the class is in another namespace, add a use statement at the top of the file:

use App\Services\Invoice;

$invoice = new Invoice();

#Regenerate the autoloader

If the class genuinely exists and is named correctly, Composer's autoload map may be stale:

composer dump-autoload

For framework classes resolved through the container (controllers, etc.), the symptom is slightly different, see Target class does not exist in Laravel.