PHP generally is used to program the back end of websites. The parts that deal with databases and users/passwords, any program that stores information. It is used for most programs on websites today, it is completely server-side, unlike JavaScript or HTML which are interpreted by the web browser on the client-side. The server reads PHP which generally tells it what HTML or JavaScript to display to the browser. When you submit a form it may be handled by PHP as well. So for any sort of interactive website, or a site that needs a database or administration section PHP programming is essential. I may be slightly bias as I am a PHP programmer. One other thing, PHP among many other uses is also used for inter-changeable templates on websites, that is with the click of a button the website's template changes.