Philadelphia. The edges of Zip Codes are slightly modified for logical and cartographic purposes.
This polygon layer has an accompanying arc layer. Certain arcs in the arc layer contain data signifying information relating it to the polygon layer. It can tell you if both sides of the arc belong to one of the bounding polygons. All the arcs, including those with no boundary info, have naming attributes for labeling the polygon borders. Contact the Streets GIS unit for public consumption of the corresponding arc layer.