PHP is a scripting language that you can use to create interactive web pages. PHP code can be embedded into an HTML page and is usually given the extension of .php when saved to a file. Some PHP pages can also contain code that accesses a MySQL Database so that your visitors can access real-time data, and maybe even add, change or delete information that can be displayed on the web page. Examples of PHP pages would be some Login forms and forums. Note that all your PHP pages must reside on your CGI Server.

For more information on php scripting, please see PHP.net official site.