Magento Autoload

lib_Varien_Autoload.php

<?php
static public function register()
{
    spl_autoload_register(array(self::instance(), 'autoload'));
}

/**
 * Load class source code
 *
 * @param string $class
 */
public function autoload($class)
{
    if ($this->_collectClasses) {
        $this->_arrLoadedClasses[self::$_scope][] = $class;
    }
    if ($this->_isIncludePathDefined) {
        $classFile =  COMPILER_INCLUDE_PATH . DIRECTORY_SEPARATOR . $class;
    } else {
        $classFile = str_replace(' ', DIRECTORY_SEPARATOR, ucwords(str_replace('_', ' ', $class)));
    }
    $classFile.= '.php';
    //echo $classFile;die();
    return include $classFile;
}