While reading up on PHP security, I came across several links that I thought I would pass on:
Security Focus article on securing PHP installations: http://www.securityfocus.com/infocus/1706
Article on PHP attack vectors and defenses: http://www.securereality.com.au/studyinscarlet.txt
Bugtraq discussion on PHP security: http://www.securityfocus.com/archive/1/438417/30/0/threaded