Load extension classes

Im trying to write a site, where I want to be able to add extensions into it at a later date. I want to loop over a folder and include each file in the folder which I can do easily. My problem is how to list all available classes that Ive loaded.

So what Ive come up with is a simple test which loads all available classes before, then reads a class, reads all available classes and then looks at the 2 arrays and displays the differences which should be just my class:-

<?php
  $startupClasses = get_declared_classes();

  class testMe {
    public function __construct() {

    }
  }

  $endClasses = get_declared_classes();

  $availableClasses = array_diff($startupClasses,$endClasses);

  print_r($availableClasses);

?>

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However all I get is an empty array back.

Anyone got any ideas how to fix this, or if anyone has any ideas how I can simply load extensions by putting files into a folder?

Thanks in advance