Any recommendations for php? I wrote a php/mysql web application for SaaS customers. I am looking at ways to improve performance/cache/error handling or simply write a better code.
For a smallish project that you can figure out fairly easily, I recommend the Kohana framework. It's extremely well documented and it's quite easy to read. Many people actually recommend reading its source to supplement the documentation and after having to do so myself I can see why.
I learnt a lot by using frameworks as symfony. The initial learning curve is steep, but then you can understand architecture better. The jump to other languages such as python and java was really easy after that.
There's a lot of excellent PHP code in the Horde Project's repository: http://cvs.horde.org/
There is no such thing as excellent PHP code.
The symfony framework and Doctrine ORM are both well written projects.
Perhaps MediaWiki?
Personally, I wouldn't recommend it. Unfortunately I can't give a better alternative, but I've done some hacking on MediaWiki since version 1.7 and while it's some of the better examples of PHP out there, it still leaves lots to be desired in different places.