PHP (Pre Hypertext Processor) is a programming language used to dynamically create HTML. It is "server side" meaning it is run on the web server and not directly seen by the web browser. The web browser sees only the resulting output.
Why use server-side programming?
Server-side programming allows us to manipulate data on the server. Some practical examples of this are:
- Reformatting existing HTML to add recurring elements to each page such as toolbars, headers and footers.
- Saving data entered into a form by a user.
- Reading and writing to files and databases.
Why use PHP?
PHP has many advantages over competing programming languages.
- Main stream popularity - there is a great deal of information, support and 3rd party applications available.
- Easy to use - PHP is a high level scripting language which makes it easier and faster to write and maintain.
- Cross platform - PHP is available on all popular operating systems.
- Free - There is no license required to develop or run PHP.
- Custom PHP development
- PHP based CMS (Drupal, Joomla, etc.)
- Maintenance of legacy systems
- Integration with 3rd party systems
- MySQL database applications
If you would like to investigate further here are a number of good resources for more information.