Can Facebook apps be made in PHP? Does anyone know any good tutorials? It could be a great way to make a little money or promote your site/product.

Ofcourse faceback can be developed in php.
Infact php is the best scripting language for developing community websites.
It may be difficult to find the whole application as a tutorial, but you can do step by step anyalysis on facebook and search for tutorials for that specific areas/functionality.
Once you get these developed or atleast understanding, you can integrate them into one big application. It's not difficult but may take some time in the begining.

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I have never come across any good tutorials or books for learning how to make apps in Facebook. But the most useful thing would be their develop forum.
Here is the link
forum[dot]developers[dot]facebook[dot]com/index[dot]php
replace [dot] with [.]