I found this info from my host 1and1. Hope it helps...
With 1&1 Linux hosting packages you have the choice between PHP4, PHP5 and PHP6. 1&1 Web Hosting packages use PHP4 as default for all .php files. For more information about PHP5, please reference What's new in PHP5?
You may choose to simply name the script with the .php5 file extension if there are only a few scripts that require PHP5. If you don't want to rename all your scripts to .php5, you can create a .htaccess file (or edit the existing .htaccess file, if you already have one). You can create this file using a text editor such as Notepad, WordPad, vi, nano, etc. and place the following line in the text file:
AddType x-mapp-php5 .php
Save the text file as .htaccess in the folder where you need to use PHP5 (or upload the file to the correct folder). The settings specified in the .htaccess file are recursive meaning that the settings will apply to the current folder where the file is saved as well as all subfolders.
While the PHP developers have done their utmost to ensure compatibility, we cannot guarantee that PHP4 scripts will function 100% correctly under PHP5. PHP5 has register_globals, allow_url_include and allow_url_fopen all disabled by default which may cause improper functionality for some scripts when switching from PHP4 to PHP5.