PHP is a open source language which is widely used in the web application with MySql database connectivity. its platform-independent engine that parse the php scripts. PHP is faster and too easy to implement in web application. PHP runs on Apache or IIS server. If you developing the website using LAMP ( Linux, Apache, Mysql and PHP) then its cost is too low because linux is free operating system.
ASP.NET is the part of .NET Framework and technology to develop the web application. ASP.NET use MS-SQL to database connectivity. Its runs on IIS server. ASP.NET provide you great security in web application.