53 Pa. Stat. § 12588
Whenever any debt shall be or shall have been created for which the Constitution of this Commonwealth requires a sinking-fund to be established, the proceeds of the taxes levied for the payment of the principal and interest of such debt and all other money pledged or appropriated for the payment of the principal and interest of such debt shall be paid into the sinking-fund of such city, and shall be inviolably reserved for, and applied exclusively to, the payment of the principal and interest of such debt.
Whenever there shall be money in the sinking-fund in respect of a particular debt in excess of the requirements for the payment, during the twelve months next ensuing, of principal maturing and interest due, such excess money shall be applied to the purchase and cancellation of such debt, but, if at any time it shall be impracticable or financially disadvantageous to purchase such debt, such excess money may be invested temporarily in bonds or other evidences of debt of the United States of America, of this Commonwealth, or of any county, city, borough, township, school district or other municipality or incorporated district of this Commonwealth.
53 P.S. § 12588