I have the perfect answer for you. Use ASP.NET there are many reasons why.
1. Faster than php because it runs on the .NET framework is based on compiled languages and the Web applications is compiled and not interpreted.
2. If you do ASP.NET use C# for the code behind pages because it is a lot colser to java than php is.
3. In using ASP.NET I have found more help for it and nicer people wereas in the open source community (that i love by the way and wil fight for anyday of the week.) I get a lot of RTFM on stuff that i dont understand even after i have read for a couple of hours on how to do it.
4. Another reason is you have a lot better IDE's to choose from for ASP.NET such as the webmatrix which is an excellent free IDE for asp.net. You can get it at http://www.asp.net also there is Visual Studio.NET that a lot of people use but it costs money, but IMO is well worth it.
5. There is honestly more money in ASP.NET if you are looking at makeing some money at it I frequently find more jobs of people looking for .NET developers for ASP.NET or window forms projects than I do php.
6. If you learn ASP.NET you will have a major heads up on learning Windows Forms to make regular applications in windows using the .NET framework.
These are just a few of the reasons why you shoule learn ASP.NET over php. I know both but personlly love how fun and easy it is to develop ASP.NET applications over php web sites.
Hope that helps.
p.s. For a great resource to learn ASP.NET go to http://www.asp.net there are lots of people there to help you out. Also for excellent articles to read on learing ASP.NET visit http://www.4guysfromrolla.com