There's lots of tools out there, and it's rare that people will actually use the best tool for the job - they'll choose the tool that best suits them, typically because of familiarity. And thats completely fine, of course. Each to their own, and all that.
The only thing that drives me mad is when people make incorrect statements about tools they clearly know little or nothing about, and pass it off as advice. hehehe
When it comes to PHP or ASP, there's not really all that much of a difference between them. They both achieve the same things in very similar ways, and there's plenty of support/hosting/advice, etc... available for both of them. So whatever floats your boat, as they say.
ASP.NET however is completely different, and really can't be compared to the others. It would be like comparing a carrot to a bicycle. There is no comparison.
For the budding developers reading this thread, if you haven't started getting into .NET yet, I really would urge you to take a good close look at it. Remembering that it's not language dependant. You can still code in PHP when your working in the .NET environment. Or ASP, or VB, or C#, or J#, or Iron Python, or Ruby on Rails, or whatever language you like, because .NET uses CLR (Common Language Runtime).
.NET is far more powerful and advanced than anything you could imagine if all you've ever seen is PHP or ordinary ASP. But for developing simple websites (or even complex ones), .NET makes it far easier too, once you've grasped the basics. And now there's also LINQ, and BLINQ to make data manipulation incredibly easy by turning your data tables into classes (very clever and powerful stuff). Microsoft have also recently released MS Ajax 1.0, making the use of Ajax controls a breeze.
At the end of the day, absolutely use whatever your happiest with. Be aware though, that with a little research you might find a better tool that you would be more happy with - don't just stick your head in the sand and stagnate because it's what your used too.