I've been looking at frameworks a lot lately. Mainly for an upcoming project, but also to getting a better understanding of MVC based development, as well as components/helpers for web development. That made be ask the question, what framework does php.net use for it's documentation/downloads site?
I read somewhere that Rasmus Lerdorf, the guy who invented php, who went to University of Waterloo, by the way, felt particularly fond of Code Igniter. I've looked at it, and heard great things mainly due to it's ease of use, but echoing Rasmus' thoughts, if feels much more like a library than a framework, at least to me.
I've been playing a lot with CakePHP lately, and that, in my mind, is a true framework. Everything is native to Cake. Their site is very intimidating, and their framework is VERY expansive, but it's therefore very powerful. I'm not sure it's exactly what I was looking for, but it's given me invaluable insight into framework development for php.
This then leads me back to the question of what php.net uses. My guess is a custom framework; not symfony, or cake, or CI, or zend or any of those guys (probably because they came about way later than php.net), but I'd be interested to know what they do use, and if they'd open source it. For now, I'll keep working on my own framework that is geared towards being lean, and requiring me to do a lot of the heavy lifting for maximum flexibility. Hopefully I'll open source it someday, and it'll be able to meet the needs of people like me for whom Cake was too much, and CI (Code Igniter) was too little.