Linux Security is another good site that is not necessarily restricted to PHP but, since you are likely running a Linux Webserver to host your PHP applications, it's useful to try to stay up to date on the latest advisories and news related to your chosen Linux distribution. Don't assume your hosting company is on top of these developments; be aware on your own -- your security is only as good as your weakest point. It does you no good to have a tightly secured PHP application running on a server with an outdated service that exposes a well-known and exploitable flaw.