i'm sure you know this but i'll say it for the sake of other readers.
you can use PHP on any kind of server, Windows, Unix, Linux and Mac (i'm sure there are other too, these are the common ones).
using ASP.NET would be constrained to just Windows based servers. Don't forget PHP is open source!
Personally i like the flexibility of having an option to move to a different kind of server, especially if you are a small company and if the hosting rates change and/or you find a cheaper one. 🙂
Also using php you have great community. There are ton of examples and free code out there, i'm sure ASP.NET doesn't have anything near this big.
Saying this, the rate of finding bugs and its fixes is, in my opinion, much faster.
Obviously, all this can be debated. 🙂
Typical to Windows vs Linux kind of discussion.
note: a lot of this is based on using ASP not ASP.NET, since I know ASP but haven't gotten around to learn asp.net. I've found php much easier to code and find helpful resources.