What is PHP?
PHP stays for Hypertext Preprocessor and is an open source server side programming language that is used for web scripts and to process data passed via the CGI (Common Gateway Interface) from HTML forms etc. PHP can be written as scripts that reside on the server and may produce HTML output that downloads to the web browser. Alternatively, PHP can be embedded within HTML pages that are then saved with a .php file extension. The PHP sections of the page are then parsed by the PHP engine on the server and the PHP code stripped out before the page is downloaded to the web browser.

What is the difference between PHP 4 and PHP 5?
There are several differences in the configuration and in the way certain functions and expressions are handled. PHP 4 has a larger installed base at this point, but since PHP is a server side script, it will hardly affect the end user. PHP 5 is simply a newer release, which offers its own improvements and add-ons. However, there might be some incompatibility issues when certain applications are run on both versions. You should use PHP 5 only if it is required by the script author.