Set your domain on Dreamhost to use PHP5 instead of PHP4.
You do this by going to Domains->Manage Domains in the Dreamhost Control Panel, and clicking the "Edit" link that appears to the right of the domain in question.
At the next screen, select the radio button for PHP5, and submit the form.
Once the setting takes effect (usually 5-10 minutes, though it could be longer) you are "good to go". You know that change has taken place when the "clock icon" next to the domain in the Manage Domains screen goes away (it indicates a change is "pending").