PHP scripts has many applications and it is used world wide. There are 3 main area where PHP scripts are used:
Server-side scripting: Server-side scripting is a technique used in web development which involves employing scripts on a web server which produce a response customized for each client's request to the website. This is the most traditional and main target field for PHP. To do the server side scripting work you need 3 things: Web Browser, Web Server and PHP Parser.
Command Line Scripting: By using PHP parser you can do command line scripting, you don't need web server or browser for this. This is used to run your task scheduler or cron jobs or text processing tasks.
Desktop Applications: Surprise, yes PHP can be used to write Desktop applications, though it is not very best with it. We use PHP GTK to write such program. PHP GTK is not bundled with main distribution.