Okay, incredibly tired so let's see if I can get this all out and have it make sense.
pyornide summed up my thoughts quite nicely, PHP can really be a pain to work with. The language is riddled with some scary design decisions.
Also, as for what one of the previous posters stated about PHP's MySQL support, PHP really loses some points right away here since almost every tutorial still seems to suggest using PHP's old MySQL driver which promotes rather dodgy and insecure ways of doing things. Just generally, you often see people citing the quality of PHP's documentation. It's quite comprehensive, sure; though it doesn't go without making some really stupid and/or outdated suggestions. The user comment system--while a good idea in theory--may hinder more than help. I've fallen out of touch a bit though, so maybe things in these areas have improved.
Perl--not exactly the newest kid on the block, but its a solid and flexible language, and the community around it is still near unparalleled. Additionally, CPAN is quite an invaluable resource, if you need anything chances are there's already a module there that'll do it.
As for ASP.NET, I haven't worked with it enough to have much of an educated opinion. From what I've experienced, however, the overall ASP.NET way of doing things--configuration, data flow, conventions--just doesn't seem quite... right; not for the web. I can't base this on anything other than my personal opinion, of course. The framework obviously works for some.
Python is a versatile language with an extensive standard library and some nice documentation to go with it. It's equivalent to CPAN isn't as big, of course, however it still provides plenty of useful third-party libraries. A while ago I ditched PHP completely for creating any new web applications. Most of the applications I deploy on the web now are running Python, atop of a variety of frameworks and utilities, and maintaining them is much more satisfying. Alone, the increased maintainability the language allows was a big factor (out of many) of it winning me over.
My two cents.