What is the test for a claim under the equal protection clause?

"'The concept of the equal protection of the laws compels recognition of the proposition that persons similarly situated with respect to the legitimate purpose of the law receive like treatment.'" [Citation.] 'The first prerequisite to a meritorious claim under the equal protection clause is a showing that the state has adopted a classification that affects two or more similarly situated groups in an unequal manner.' This initial inquiry is not whether persons are similarly situated for all purposes, but 'whether they are similarly situated for purposes of the law challenged.' [Citation.]" (Cooley v. Superior Court (2002) 29 Cal.4th 228, 253.)

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