We build quite big systems (many years of manpower) on top of Linux, Apache, MySql and PHP and we are very happy and successful with it.
We develop our own framework.
I would recommend Zend Framework. I have had issues with Code Igniter. All the workarounds and unexpected stuff frustrated me - I think not only me but other users as well - that is why I think there is a fork: http://kohanaphp.org ... By the looks of it, I would recommend Kohana instead of Code Igniter. Either way, I am not so sure about the future of CI. That is why I choose Zend Framework.
The proper link to the KohanaPHP project is: http://kohanaphp.com. I've been using Kohana for the past few months and it is major improvement over other PHP MVC frameworks. It utilizes the OOP functionality in PHP5 to make structured PHP web development much easier and elegant. If you are looking for a new PHP framework, you should definitely consider Kohana. You could even integrate with Zend Framework classes if needed.
that site seems to be under construction...?
i prefer codeigniter as a good php framework.
benchmarks: http://tinyurl.com/5dcjzd
For the good of your soul, use a framework. We currently like Zend Framework, but I've heard good things about CakePHP and Symfony. If you stick with the framework and follow a sane MVC strategy you'll fix most of the problems you'll have using PHP as an HTML templating language.