PHP spl_autoload_register returns error

[Roy Harper]

Hello. I am trying to use spl_autoload_register but it returns an error saying "Uncaught Error: Class 'Functions' not found in..." Here is my code:

spl_autoload_register(function($className) {
    $path = "classes/";
    $extension = ".php";
    $fullPath = $path.$className.$extension;
    
    if(!file_exists($fullPath)){
        return false;
    }

    require_once $fullPath;
});

$fnObj = new Functions();

When I try removing the

if(!file_exists($fullPath)){
    return false;
}

there is no error. What could possibly the reason why this is happening?