PHP translates into Hypertext Preprocessor in the long form. This open source scripting language is a widely-known programming platform used in developing dynamic & interactive web pages & applications. So all PHP code is translated by a web server containing a PHP processor module to produce the resulting web page. Although it is server-side scripting language, it can also be used as a generic coding language.
So in short, PHP is a useful platform to learn if you want to tackle coding on a cross-system environment, as is the case with almost all web applications today. So when are you learning PHP?