If we're talking PHP vs ASP 3.0, from my experience, I'd choose PHP over ASP easily. It runs smoother, is easier to debug, is free, and implements itself very easily with both IIS and Apache.
However, if you're talking ASP.net, I'd take it anyday over PHP. I have my personal reasons, but as a business application developer (both Web and Windows based applications) its just makes everything much faster in development speed and functionality, as well as full Object-oriented programming. Yes, there is an implementation of OOP in PHP, but not as powerful or structured as that of ASP.net (being that ASP.net's code-functionality is based on VB.net/C#). Plus debugging has been made very simple with it, and it implements well with most databases via ADO.net (including MySQL with a little work). Yet, like anything Microsoft does, there is a fee involved (as IIS is only shipped with Windows 2000/XP/2003 Server) and in order to get the most functionality out of ASP.net, you'll need Visual Studio 2003+ (2002 just doesn't cut it as it doesn't have full ASP.net 1.1 support, only ASP.net 1.0).
This is not to knock PHP though. I have programmed extensively with it in the past, and it is a very powerful web-scripting language. Anything you can do in ASP 3.0 you can do in PHP, and then some. I guess it boils down to whether or not money is somewhat of an issue. If not, I'd look into ASP.net (not ASP 3.0). MS have also released a free slimmed down version of Visual Studio just for ASP.net called WebMatrix (
www.asp.net) I haven't used it myself, but it might something worth looking into.