No person holding a position in any city as a laborer, at a per diem wage, shall be compelled to pay or contribute toward the fund herein provided for, but he shall have the option or choice of so doing, and shall only, upon electing to contribute to the fund, become entitled to the compensation provided by this act: Provided, however, That he shall be required to contribute three (3) per centum of his wages and the same percentage upon any amount of compensation he receives after his retirement.
53 P.S. § 42006