It's kind of hard to compare them for a few reasons. The first is that there's only so much you can do on the internet from the server side of things and both languages do this very well. With cookie handling, http requests, all of these kind of things. But it's the way in which these things are handled which seem to be the issue for most people. There are lots of arguments about why PHP is a bad language, there's a very popular thread on stackoverflow entitled "Defend PHP; convince me it isn’t horrible". And these are the very valid arguments he makes... However the best argument would be that it's so easy to use. Anyone can pick up the basics of PHP and start writing scripts very quickly making it the choice for many developers. Which is great, but this often promotes bad coding/design. Would you critisise PHP for that As I've never used ASP before I can't really comment on that but from what I have gathered is a better designed language and this would encourage better coding and therefore better scripts.
PHP rules..its very easy to use. supported by all servers. free open source. it has wide range of extensions and libraries you can use. very powerful community.. and you can develop windows/across platform desktop applications with it.. plus you can use .net technologies with php there are many libraries can do this for you. you even can use silverlight with php... its simple.. php is the best language..
you can get help very very easy with PHP. If you can not do something or if you do not understand something there are hundres of people happy to help you. asp is not same. There is no help at all. you have to do by your self.
You can't really compare two diffrent programming laungages. They are useful for each project, it's about what you enjoy most to code with.
php compare with. net I think is very difficult, this of course depends on which one controlled by a programmer, I think if a programmer is php programmer, then he would say that php is better, as well as vice versa, but if we see progress, namely in terms of open source, asp.net which as we know it is the scripting of the course microsoft offsource then we can say that much better php asp.net