Did you know that Yahoo Developer Network had a special section for PHP developers?

Here’s what they have at the PHP Development Center

• HOWTO Articles to help you understand our technologies and how you can use them better with PHP.
• Code Samples you can play with. Use them as a starting point for your own applications or study them for subtle nuances.
• Other Resources out on the Web to help you use PHP and the Yahoo! APIs to their fullest.
• Community Resources where you can join our mailing list and discuss the Yahoo! APIs with us and with other PHP developers

Link: Yahoo! Developer Network - PHP Developer Center