Google App Engine与__autoload存在问题

Yesterday I started developing on the Google App Engine platform for the first time. After running into some minor newbie issues which I could easily resolve using the documentation, I now seem to have run into a trickier one. For some reason the following __autoload function doesn't work when I run it in a GAE sandbox.

function __autoload($classname)
{
    if(file_exists('system/core/'.$classname.'.php'))
    {
        include('system/core/'.$classname.'.php');
    }
    else
    {
        echo 'The class "'.$classname.'" could not be found!';
    }
}

The file does exist and when I use my normal LAMP stack it has no issues. Anyone have any experience with this?