What is the test for establishing that a state classification that affects two or more similarly situated groups in an unequal manner?

'is a showing that the state has adopted a classification that affects two or more similarly situated groups in an unequal manner. [Citation.]' [Citation.] 'This initial inquiry is not whether persons are similarly situated for all purposes, but "whether they are similarly situated for [the particular] purposes of the law challenged." ' " ( People v. Guzman (2005) 35 Cal.4th 577, 591-592, 25 Cal.Rptr.3d 761, 107 P.3d 860, italics added.)