What are the requirements for a U.S. citizen to obtain permanent residency in the Philippines ?

You can retire there, if you are over age 50 (or close to it). Check also the Philippine Retirement Agency. I went there in 2010 and visited with the agents at their office in Makati. These are good people who are eager to get you to live comfortably in their country. It makes sense as you will spend your pension and help the economy. So far in my searches I have found only Mexico and the Philippines with government agencies helping retirees choose their country for a retirement destination. Good luck! I think you made a wise choice. I love the Philippines also :o)