I'm including a PHP class with

require_once($ENGINE."/classUser.php");

but when the code is executed i receive this error:

Fatal error: Class 'User' not found in C:\xampp\htdocs\WebName\resources\engine\ajax\signup.php on line 12

I still can't figure out what's the problem. I'm 99% sure it's correct.

The "$ENGINE" is correct, and the class is correct too (Netbeans suggests me class methods and variables).

signup.php:

createUser($_POST["username"], $_POST["email"], $_POST["password"]);


?>

classUser.php:

set(
                "user.".$username,
                array(
                    'ID' => uniqid(),
                    'Username' => $username,
                    'Email' => $email,
                    'Password' => $password
                )
        );
    } 
}

?>