Vikram Vaswani, Founder, Melonfire, have written an interesting article at IBM developerWorks on using the Youtube API with PHP. Notice that a PEAR package Services_YouTube is available if you are looking for a library to use Youtube API with PHP, but is alpha since 2006.
The article introduces the YouTube Data API, demonstrates how you can use it to browse user-generated video content; access video metadata, comments and responses; and perform keyword searches. The author used the power of SimpleXML to parse Youtube data and build customized PHP applications.
