When you visit web sites at which you can sign up for a membership by registering so that later you can sign in and participate in activities offered by the site, PHP is one of a number of languages in which those pages might be written. Your registration information, username, password, etc. are processed and stored in a table in a database. Forums, calendars, and even some simple web-based games are written in PHP.
You might want to begin scanning the file extension of the web pages you visit in order to get an idea of the sites that make use of this technology. Keep in mind, however, that not all pages that are truly written in PHP actually end in the .php file extension. The use of other extensions is often for the purpose of hiding the real scripting or programming language name from the public among which malicious hackers hide.
The article, "What Are the Benefits of Using PHP?" focuses on why this language has grown to enjoy international popularity. If you're interested in learning the language, you might want to read, "PHP Tutorials for Dummies."