As everybody and his dog can program in PHP nowadays, and -- worse ;) -- does without the proper programmer training in input validation and such things, here are some tips about hardened PHP, modsecurity, putting Apache in a chroot jail etc:
Use security on multiple levels. Code your stuff so it's invulnerable, but make it so even if it isn't, the site doesn't collapse. Put your administrative settings in a different database, with different permissions, to your user stuff - if possible using the administrator's password as the (my|pg)SQL user password, so sql injections don't kill the site, just userland data. Don't have write access where it's unnecessary, or read for that matter: set permissions properly. Store SQL connections strings and other sensitive information in a file to be require_once()d from outside the web-accessible directory, just in case php suddenly becomes uninstalled. Use php_flag and php_value in .htaccess, in case your host changes php.ini without telling you. Code so you don't rely on anything within php.ini. Initialise all variables before you use them, access user input all within the $_GET, $_POST, $_COOKIE, $_FILE superglobals.
Also configure your system properly. Keep up to date! Use hardened php, modsecurity and apache in a jail for a start. Give your php user limited access only to those files it needs access to. And then it goes lower - secure your kernel. use ACLs.
The key point here? There are more entranceways to your server than you think. Block them all. Secure your system in every place possible. Use encryption. Never, ever, ever rely on a user, or administrator, for security - have everything coded securely. A good administrator will keep it that way and add their own restrictions... but a bad one might not. Also don't be slack once you get into the admin area. just because it's only meant to be accessed by someone with a vested interest in the site, that doesn't mean that's how it'll happen. Some stupid admin will use "d34db33f" as a password and think they're funny - don't let this compromise the site. And at all costs, for the sake of humanity, make it so a site compromise can't turn your server upside down, can't make your server a spammer, and can't modify anything on the server other than that specific site.
- Read the whole PHP security chapter http://www.php.net/manual/en/security.php
- Read about XSS, CSRF, SQL injections, session hijacking etc.
- Always initialize your variables
- Always escape anything going into an SQL query (if you don't use prepared statements)
- Do not use shell commands. Be very careful if you need to anyway
- Never assume anything about input coming from the user.
- Do not display data coming from the user before you are 100% sure that the data is cleaned from XSS etc.
- http://phpsecurity.org/ (perhaps)
- Do not give in to living a happy life :-) Stay cautious
- This list is incomplete.