What is the test for determining the validity of a statute?

'With reference to general principles of constitutional law, judicial decisions abound with declarations to the effect that all presumptions and intendments favor the validity of statutes; that mere doubt by the judicial branch of the government as to the validity of a statute will not afford a sufficient reason for a judicial declaration of its invalidity, but that statutes must be upheld as constitutional unless their invalidity clearly, positively, and unmistakably appears.' (People v. Superior Court (1937) 10 Cal.2d 288, 298, 73 P.2d 1221, 1226.)