Just thought I'd react to simast at gmail dot com's note: While he has a point in saying C outperforms PHP, his suggestion is micro-optimization. I'm not 100% against micro-optimizing code, but if you do, go all the way:

<?php

    // Your custom class dir
    define('CLASS_DIR', 'class/')

    // Add your class dir to include path
    set_include_path(get_include_path().PATH_SEPARATOR.CLASS_DIR);

This adds the include path to THE END of the paths PHP will scan for the class file, resulting in a bunch of misses (file-not-found's) before actually looking into the CLASS_DIR.
A more sensible approach, then would be to write

    set_include_path(
        CLASS_DIR.
        PATH_SEPARATOR,
        get_include_path()
    );