53 Pa. Stat. § 30558
Each city officer may, and each city employe shall, be required to become a member of the retirement system established by this act at the time the ordinance providing for the same becomes effective, and thereafter when first becoming a city employe. Those becoming members who have been at any time city employes prior to the first Monday of January of the year the retirement system is established, shall be known as original members, and those becoming members after said date as new members. Any person who becomes a city employe subsequent to the time the system was established, and who had been an employe of the city at any time theretofore, shall receive credit for the service prior to the time the system was established and be known as an original member, if said employe shall contribute to the fund the amount which he would have contributed if the retirement system had been in effect during such previous employment, in which event, the city shall contribute for each such employe the amount which it would have contributed if the retirement system had been in effect during such previous employment.
53 P.S. § 30558