If you "mess it up", you can easily revert back to DH's PHP install by removing the information you entered into your .htaccess file.
You'll know you messed it up, because your php-based web pages won't work. But if you follow that guide as it says, you should have no issues
If you happen to have issues with the install process, for whatever reason, feel free to post back here and someone may be able to offer you assistance (including myself).
I plan to offer a service to DH customers soon doing just that - installing PHP + options for them