From the Knowledge Base:
We have two versions of PHP installed on all of our servers. By default, it is PHP version 4.4.1. There is nothing extra to do to use this version.
To use PHP version 5.1.1, you have two choices.
1. Make sure your file ends in .php5
2. Add the following line to your .htaccess file to have all .php files using PHP5.
AddHandler application/x-httpd-php5 .php