You can roll back to PHP4. Check DH panel --> Domains --> Manage Domains --> Web Hosting --> Edit.
If you want to roll back older version of PHP5, I think you have to install by yourself. You can refer to this article http://wiki.dreamhost.com/index.php/Installing_PHP5. If you are confident to intall by yourself, you can try that.
Save [color=#CC0000]$97[/color] (max discount) on dreamhost plans by using promo code: [color=#CC0000]97CRAZY[/color]. Sign Up NOW