Dr. Phil at Gay Wedding -- Walks Interior Designer Down the Aisle

Dr. Phil played a supporting role at a wedding over the weekend ... proudly walking the groom down the aisle at a same-sex ceremony.

Neal Wagner and Thomas Piscitello exchanged vows Saturday at Vibiana -- formerly a Catholic church in downtown L.A. -- the guests included Dr. Phil, his wife and his sons.

Sources tells us Phil is both a friend and a client of the couple -- both interior designers. We're told he happily stepped in for Neal's father, who passed away, and agreed to escort Neal down the aisle.

One wedding guest said the ceremony was like the gay wedding in "Sex and the City"... over the top, but amazing.