Server-side scripting

This is the most traditional and main target field for PHP. You need 3 things to make this work: the PHP parser (CGI or server module), a web server and a web browser. You can access the PHP program output with a web browser, viewing the PHP page through the server

Command line scripting

You can execute a PHP script without a server or browser; the only thing you need is the PHP parser. This method is perfect for scripts regularly run using cron (on *nix or Linux) or Task Scheduler (on Windows). These scripts are also suitable for basic text processing tasks

Writing desktop applications

If you know PHP very well, and would like to use some advanced PHP features in your client-side applications you can also use PHP-GTK to write desktop applications. You also have the ability to write cross-platform applications this way