This is a matter of opinion. If you ask 5 .NET Developers and 5 PHP developers which language you should use, you are probably going to get 5 answers to use .NET and 5 answers to use PHP.
You said you have experience with PHP, that is probably your best bet since you know what the syntax looks like and are accustomed to working with it. Also, based on your requirements, this is extremely simplistic and each language handles these types of things simply.
Both languages, PHP and ASP.NET, support WAP and WML so designing a web based version of your application shouldn't be to troublesome.
I can't speak for the PHP community here as I don't go into the PHP threads but the .NET community here is extremely bright and, if that is the way you choose to go, I am sure someone will be able to give you a hand with any problems you may have.
hth and good luck.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
Technical Editor for: Professional Search Engine Optimization with ASP.NET