If any person shall have served twenty (20) years and voluntarily retires he shall, by continuing his contributions until the age of fifty-five (55), be entitled to the above compensation.
During the lifetime of any such person he shall be entitled to receive as compensation annually from the fund set aside for the purpose, fifty (50) per centum of the amount which would constitute the highest average annual salary or wages which he earned during any five (5) years of his service for the city, or which would be determined by the rate of the monthly pay of such person at the date of retirement, whichever is the higher. Said compensation to be paid in semi-monthly payments.
The retirement board may authorize any such member to make the election herein provided at any time, and if made prior to retirement, such member shall, in addition to any lump-sum payments required, pay to the board contributions on his entire salary or wages thereafter received at the rate provided in section 10.1 of this act for monthly salary or wages in excess of that on which social security allowances are payable.
53 P.S. § 42004