Simple php autoloader. Checks for constant 'CLASS_DIR' that specifies base directory for classes, otherwise will simply look in PSR-0 style directory structure in current directory.

autoload.php

<?php
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// From all I can find on it - PSR states class methods must have opening brace on new line - not plain ol' functions //
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
function autoload($className) {
    $className = ltrim($className, '\\');
    $fileName  = '';
    $namespace = '';
    if ($lastNsPos = strrpos($className, '\\')) {
        $namespace = substr($className, 0, $lastNsPos);
        $className = substr($className, $lastNsPos + 1);
        $fileName  = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    }
    $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';
    if (defined('CLASS_DIR') && !empty(CLASS_DIR)) {
        $fileName = CLASS_DIR . DIRECTORY_SEPARATOR . $fileName;
    }

    require $fileName;
}
spl_autoload_register("autoload");