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.