When 1&1 first offered the alpha/beta stage of the newest PHP version to its customer base, the final release was planned to be PHP6. Since that time the PHP developers have reevaluated their planned releases and have pushed off PHP6 for some time. Most changes planned for PHP6 have already been incorporated into version 5.3 and 5.4 as there is some uncertainty to when PHP6 can realistically be released. It is because of this reason that the extension .php6 is used to enable PHP 5.4.
With 1&1 Linux web hosting packages and website server packages, you have the choice between PHP 5.2 and now PHP 5.4. To parse a single page as PHP 5.4, simply name the page with the file extension .php6. You can also Change the Default Version of PHP for your Entire Package.