All contracts and agreements heretofore made by the county commissioners of any county for the improvement or maintenance of any such public highway in the manner authorized in the foregoing section are hereby validated and made binding upon the county, township, borough, or incorporated town, as the case may be, as if full legal authority existed therefor at the time that such contract was entered into; and are hereby declared to be as sufficient in law as if the same had been made after due notice published by the controller of the said county after approval by him of the purpose of the proposals; and as if bids had been received by the controller under seal and in his presence opened by the commissioners and the contracts awarded; and as if the same had been made after due advertisements and receipt of bids by the authorities of every county, township, borough, or incorporated town which is contributing to the cost of said improvement or maintenance.