Loader.php

<?php

class Loader {
  private static $loadPaths = array();

  static function autoload($class){
    // assume its name is its path (ActiveRecord_Relation_BelongsTo is in ActiveRecord/Relation/BelongsTo.php)
    $fileName = str_replace('_', '/', $class). '.php';

    foreach (self::$loadPaths as $path){
      if (file_exists($src = $path . '/' . $fileName)){
        require $src;
        return;
      }
    }
  }

  static function addPath($path){
    self::$loadPaths[] = $path;
  }

  static function startAutoLoadingFrom(array $loadPaths){
    self::$loadPaths = $loadPaths;
    spl_autoload_register(array('Loader', 'autoload'));
  }
}

usage.php

<?php

Loader::startAutoLoadingFrom(array(
  ROOT_DIR . '/app/models',
  ROOT_DIR . '/app/controllers',
  ROOT_DIR . '/lib',
  ROOT_DIR . '/vendor'
));