Stuck, big time

I'm a professional webdesigner, but a recent job i recieved asked me to host the webpage on my computer, which means i'm trying to install php on my own system. I have done so, but when opening a .php file i see the source code and it's ignoring that php exists. If i stick <html> on the start to make it an html page it translate the html parts into being visible properly, but the php parts are still ignored. What am i doing wrong? Thankyou David Lurie