I followed the YouTube API - Developer's Guide: PHP, yet I got this error running their code, don't know why they are not updating the guide.
Fatal error: Class 'Zend_Uri_Http' not found in /home/path/public_html/ZendGdata/library/Zend/Gdata/App.php
As the error show us the fix is really simple, just load the Class 'Zend_Uri_Http'
require_once 'Zend/Loader.php';
Zend_Loader::loadClass('Zend_Gdata_YouTube');
$yt = new Zend_Gdata_YouTube();
Zend_Loader::loadClass('Zend_Gdata_AuthSub');
Zend_Loader::loadClass('Zend_Gdata_ClientLogin');
Zend_Loader::loadClass('Zend_Uri_Http');
2010/08/14 Note: you have to load this class in your page where you call the YouTube API service (not in any zend php client library files.)