Comparison of PHP to ASP.NET is pretty clear. Compared to the power of ASP.NET, especially those due to the power of .NET, PHP is just nothing. Really rudimentary. PHP language is not a decent OOP language, just a collection of the irregularities.
Many recommended not to use PHP as a first programming language, as it can kill the test to good programming practices forever, in great contrast to C#. Some say, young people get into PHP programming because it's easier to start and later have their careers dead-locked with it. Fortunately for them, PHP programming is in considerable demand right now, but how about future?
There is
only one benefit of PHP: price. Right now it's very popular, so it made possible to use it as a mass production tool and drop the prices of hosting compared to ASP.NET and anything else. PHP is a must-have component of LAMP bundles.
(See
http://en.wikipedia.org/wiki/LAMP_(software_bundle)[
^].)
It is good enough for simple sites which require little maintenance. That's why I use it in my personal Web site.
[EDIT]
Another apparent PHP benefit I forgot to mention: multi-platform. Thanks to Peter_in_2780.
ASP.NET also can be used on different platforms with Mono and Apache module mod_mono, see
http://en.wikipedia.org/wiki/Mono_(software)[
^] and
http://en.wikipedia.org/wiki/Mod_mono[
^].
—SA