A project I'm about to start on will require me to build a web service in PHP to grab & return data for/from a Flex-built front end. I've got a decent amount of experience in PHP and programming in general, but I haven't done anything with XML, which this would require. The Flex app passes in XML to the PHP app, the PHP app query's a MYSQL DB and returns XML to the flex app. Pretty standard, but I'm a newbie when it comes to using XML & PHP, and with building PHP "web services" in general.

I've got lots of PHP + MySQL experience, so I don't need any help on THAT front.

Do you happen to know of any books, or free web articles/tutorials that could guide me through he beginnings of that? I'm a pretty quick learner, but guidance at this stage would be great. I'm not specifically responsible for the Flex app, but I do need to learn all that as well. Links to resources on "all of the above" would be great if any of you would be so kind.