There is no published API for interfacing with the DreamHost email account/forwarding set-up processes that are available from the control panel.
That doesn't mean it can't be be done; only that it can't be done easily. If you search these forums you will find example code where others have implemented some degree of "control" (for instance, the ability to create an address under a DreamHost account that forwards to an existing email box on Dreamhost) via PHP. You could always use that code as a "starting point".
It's hard to say more as I don't really know what functions/properties of the "forwarded email address" you are talking about when you say "controlled by php" (creation, deletion, editing, password change, change where it forwards to, etc).
At any rate, doing any of that would be an unsupported programming project that would likely break if certain changes were applied to the DreamHost panel in the future - you could hack something together, but without an established API, you would have no way of knowing if it could be expected to continue to work for any period of time.