During the development of PHP5, care was taken to change the code as little as possible. Therefore, the transition from PHP4 to PHP5 is much easier than the transition from earlier versions. For more information on the most notable changes, please reference PHP4 vs. PHP5
Most of the PHP scripts should run with little or no modification when using PHP5. To ensure a smooth transition however, we recommend testing the execution of your PHP code as PHP5.
To do this, it is suggested to Change the Default Version of PHP in the Control Panel to PHP 5.2 or PHP 5.4. It is suggested to do this during a time where your website receives the least amount of traffic. You can then test how the site reacts to the new version of PHP and assess whether any modification will be needed. If there are severe issues with the website using PHP5, you can revert to PHP4 in the Control Panel. Please be aware that starting August 1st 2013, PHP4 will no longer be offered by 1&1 and all websites will be parsed using PHP 5.2.