I know that both DXM and PCP are classified as dissociatives, but I think that's as far as their relation goes. I doubt that Dextromethorphan would come up as a false positive for PCP, because then anyone who took cough medicine in the past week would look like a drug addict. I'm not sure though...

"DXM is not specifically tested for in drug tests, but there is speculation that it could trigger the pcp test which is standard."

Speculation, but I haven't found anything to suggest that it definately does. Just list that you've taken cough syrup in the past week for a cold, DXM isn't illegal:

http://www.erowid.org/chemicals/dxm/dxm_testing.shtml