53 Pa. Stat. § 22195
In all cities of the second class of this Commonwealth, the mayor thereof, in addition to the powers now possessed, is hereby authorized and empowered, in case of his necessary absence from the city or his illness or other temporary disability, to designate, in writing filed with the city clerk, one of the heads of the city departments, except the city controller, in such cases to act as mayor; and the person so designated shall be known as deputy mayor, and shall exercise all the powers and discharge all the duties of mayor during said absence, except the power of appointing to or discharging from office in the city government; such head of department shall serve without additional compensation; and such deputy mayor shall give bond, to be approved by the council, in such sum as it shall fix, said bond to be deposited with the city controller.
53 P.S. § 22195