To summarize it, I would say PHP is the "get shit done" language. It allows people who are just starting out with programming or people who aren't very technical to focus on what they need to have, rather than on the best way to do something. This is why so many websites (Flickr) and OS pieces of software are in PHP - they focused heavily on what users would be seeing, rather than what developers would be seeing and won out.
Of course, PHP doesn't preclude good design either. Once you have a working product that is excellent on the outside but ugly on the inside, you can start spending more time refactoring your code, or even slowly migrating your system piece-by-piece to another platform.
I don't mean to be a jerk here, but just saying why something is good doesn't mean it wins. Lots of languages are good and I could list lots of good things about lots of languages that would make <i>them</i> win.
Of course, PHP doesn't preclude good design either. Once you have a working product that is excellent on the outside but ugly on the inside, you can start spending more time refactoring your code, or even slowly migrating your system piece-by-piece to another platform.