53 Pa. Stat. § 15781
Whenever any city of the first class shall be authorized, or shall heretofore have been authorized, by the municipal authorities thereof to incur debt on behalf of such city, for the raising of funds for the construction of any public improvement, or for other municipal purposes in pursuance thereof, it shall be necessary or proper for such city to execute or enter into a contract or contracts, it may and shall be lawful for the councils of such city to make the necessary appropriation, and for such contract or contracts to be executed and entered into, and work to be done and material to be furnished thereunder, without awaiting the actual issue of the loan or loans so authorized or the receipt by the city of the money to be borrowed: Provided, That the purpose or purposes of such loan shall be, or shall have been, designated in the ordinance directing or permitting the debt to be incurred.
53 P.S. § 15781