77 Pa. Stat. § 442
Every officer of the Commonwealth of Pennsylvania, or any bureau or department thereof, or of any municipality, or any bureau or department thereof, or any municipal division or subdivision of this Commonwealth, who shall sign, on behalf of the said Commonwealth or any municipality thereof, or any municipal division or subdivision thereof, any contract requiring in its performance the employment of labor, shall require, before the said contract shall be signed, proof that the said contractor with whom the contract is made shall have accepted the Workmen's Compensation Act of 1915, and any supplements or amendments thereto which may be hereafter passed, and proof that the said contractor has insured his liability thereunder in accordance with the terms of the said act, or that the said contractor has had issued to him a certificate of exemption from insurance from the Bureau of Workmen's Compensation of the Department of Labor and Industry.
77 P.S. § 442