Consider setting up
open_basedir on a "per site" basis.
open_basedir is a php.ini setting which will prevent your scripts from accessing files outside of a defined white list. If your server hosts several sites, it'll prevent one site from reading the database settings from another site. It'll also prevent a php script from accessing/modifying core system files. Open basedir is easy to set up, just add the line "
php_admin_value open_basedir /my/list/of/folders:/as/a/colon/seperated/list" to each Apache vhost.
Also consider turning off the PHP script engine for all sites/folders which shouldn't contain PHP scripts (e.g. an uploaded images folder). Again, this is simple, add "php_admin_value engine off" to any Apache VirtualHosts not needing php. To disable PHP in a directory, put the same thing into a Directory tag.
Run file permissions as tight as possible, avoid write access to PHP scripts for the Apache user, this prevents a running script from modifying itself or other scripts on the same site/server. Avoid 777 permissions if at all possible, figure out the minimum permissions required to run the application and use those.
If you're hosting multiple sites, each with their own database, use a seperate MySQL/Postgres user for each, and set permissions on each user such that they only have access to the relevant databases. Again this will prevent a rogue script from tampering with another application's database.
Suosin, HardenedPHP, mod_security and the like are all valuable as well, but use them in addition to a tightly locked down configuration, not instead of.