What CMS package are you running ?
php version archive - back as far as 3.0.18 at least..
http://www.php.net/releases/
1. Personally I would back everything up (CMS and Database)
2. Setup a development box matching the old environment and restore the backup. ( since you will have to compile each version of PHP necessary)
3. Do the upgrade through the various versions on the development machine
4. Once you get to the current version and everything is working, back it all up again and move it to the production box..