Notwithstanding the provisions of any general, special or local law, any district may submit to the attorney general any contract or agreement, or any bonds issued under the contract or agreement entered into pursuant to the provisions of this article after the bonds have been prepared, signed and sealed. Upon submission to the attorney general, he shall examine into and render an opinion upon the validity of the contract or agreement or of any bonds and the regularity of all proceedings in connection with the execution and issuance thereof. If the contract, agreement and bonds have been executed and issued in conformity with the constitution and laws of the state, and are binding, legal and valid obligations of the district and enforceable according to the terms thereof, the attorney general shall certify in substance upon the back of each of the bonds, and on the back of any contract or agreement or of a certified copy thereof so submitted to him, that it is issued or executed pursuant to the constitution and the laws of the state. The certificate shall be conclusive evidence of the authority of the district to enter into the contract or agreement or to issue the bonds and of the validity thereof.