Refers to the fact that if you live in Philadelphia for more than a few months someone will steal from you or randomly smash something of yours (ex. car windshield). Once you have been stolen from, or had something of yours pointlessly destroyed by some drunk, thug, retard, ghetto (are you my daddy?) bastard, punk, etc. Philadelphian you have paid the Philadelphia tax. The Philadelphia tax is often paid many many times throughout a lifetime.
Person #1: "Someone smashed my window after the Iggles won, yo."
Person #2: "Philadelphia tax, yo."