Here's some quick information from an unbiased, informative article that discusses the differences between PHP and ASP.NET.
According to the article, PHP 5 absolutely destroys ASP.NET. It seems that PHP (a dynamically typed, interpreted language) not only runs faster than ASP.NET (strongly typed, compiled language), but it's actually more efficient with it's less robust state management utilities. And it's free! Who'da thunk?