Currently I am just working on a website this is simple HTML website .I am thinking to increase the functionality of this web site .Please suggest me , which language should i prefer have option in between ASP.net or php ?

WHY PHP?

PHP is open source, hence there is more freely downloadable staff on the net.
PHP can run on the linux shared hosting servers, which are cheaper to purchase.
PHP is more stable and is available for more operating systems.
It runs faster

Why ASP.NET?

Variables in PHP are not really considered to have a type, and type checking is very loose, which can potentially lead to problems
ASP.NET Visual Studio, Intellisense, Debugging in VS, and other tools in this IDE makes development easier, providing that you development computer can handle it
Applications can be written using various programming languages (e.g VB.Net, C#, J# ...)