WP:Class Autoloading

functions.php

<?php
/**
 * Autoloads the requested class. PSR-0 compliant
 *
 * @since 1.0
 * @param  string $classname The name of the class
 */
public static function autoload( $classname ) {

  $filename = dirname( __FILE__ ) .
    DIRECTORY_SEPARATOR .
    str_replace( '_', DIRECTORY_SEPARATOR, $classname ) .
    '.php';
    
  if ( file_exists( $filename ) )
  	require $filename;

}