PHP stands for 
"PHP: Hypertext Preprocessor".  It is what's called a server-side scripting language.  This means it runs on a webserver you might access when browsing the internet.  Lots of sites use PHP because of it's security, ease of use and the community that supports it.  
You cannot run a .php file without special software which is installed on a webserver.  Most people do not have this software on their computers at home, but the x10 servers have PHP enabled, so if you have a .php file you would like to run, you can request an account and start creating a website with PHP here.
If you would like to learn more about PHP check out the FAQ section of the official PHP site at: 
www.php.net/faq.php