Your international Envoy boarding pass should get you club access same day stateside anywhere along your path to PHL. May want to print the rules prior to travel and carry with, the DCA staff is pretty sharp but can't hurt being prepared. Be sure to turn right after entering the PHL Envoy club - better food/drink than self-serve on the left.

One more tip - Legal Seafood, just before DCA security has some of the best airport dining in the US Air network. Enjoy.