71 Pa. Stat. § 779.4
In the event that the Governor, for any of the reasons specified in the Constitution, is not able to exercise the powers and discharge the duties of his office, or is unavailable, and in the event the Lieutenant Governor and President Pro Tempore of the Senate be, for any of the reasons specified in the Constitution, not able to exercise the powers and discharge the duties of the office of Governor, or be unavailable, the Speaker of the House of Representatives shall, if the preceding named officers be unavailable, exercise the powers and discharge the duties of the office of Governor until a new Governor is elected and qualified, or until a preceding named officer becomes available: Provided, however, That no emergency interim successor to the aforementioned offices may serve as Governor.
71 P.S. § 779.4