There are several specificities 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, this will hardly affect the end user. PHP 5 represents a newer release, which offers some improvements over PHP 4 and a few add-ons. Although the differences between the two versions are not significant, you might run into some incompatibility issues if you run certain applications on both versions. You should use PHP 5 only if it is required by the script author.