16 Pa. Stat. § 4404
If any vacancy shall occur in the office of district attorney, either by death, resignation, removal from office or from the county, or otherwise, the first assistant district attorney shall, upon a showing that the first assistant district attorney satisfies the requirements of section 1401 of the act of August 9, 1955 (P.L. 323, No. 130), known as The County Code, become district attorney and discharge the duties of the district attorney until the first Monday in January following the next municipal election occurring not less than ninety days after the occurrence of the vacancy. If the first assistant district attorney is unwilling or unable to serve or does not satisfy the requirements of section 1401 of The County Code, the judges of the court of common pleas shall fill the vacancy by the appointment of a competent person who satisfies the requirements of section 1401 of The County Code to fill the office until the first Monday in January following the next municipal election occurring not less than ninety days after the occurrence of the vacancy.
16 P.S. § 4404