ファイルがそのように構成されている場合、次のように動作するはずです
./ ( == c:\workspace\testing\ )
./index.php
./library/
./library/Zend/
./library/Zend/GData/
./library/Zend/GData/YouTube.php
./library/Zend/GData/YouTube/...
index.php
// We need 'Zend' folder to be 'level 1' child folder of a include_path directive
ini_set('include_path',
ini_get('include_path') .
PATH_SEPARATOR .
dirname(__FILE__). DIRECTORY_SEPARATOR. 'library');
// Option 1
require_once 'Zend/Loader/Autoloader.php';
$gdata = new Zend_GData_Youtube();
// Option 2
require_once 'Zend/Loader.php';
Zend_Loader::loadClass('Zend_Gdata_YouTube');
$gdata = new Zend_Gdata_Youtube();
// Option 3 (with no include_path set)
require_once 'Zend/Loader.php';
$dir = dirname(__FILE__) . DIRECTORY_SEPARATOR . 'library';
// backwards, recursively repeat loading classes which are dependencies of Gdata module
// foreach ( requestedModule::depends as $loadme ) // PS: bogus code
// loadClass($loadme, $dir)
Zend_Loader::loadClass('Zend_Gdata_Media', $dir);
Zend_Loader::loadClass('Zend_Gdata_YouTube', $dir);
$gdata = new Zend_Gdata_Youtube();