I thought I used to know, but now I’m not so sure.
See these recent studies:
Michael Wade Martin and Bryan A. Nash, “Philippians 2:6-11 as Subversive Hymnos: A Study in Light of Ancient Rhetorical Theory,” JTS 66.1 (2015): 90-138.
This study addresses the unsettled question concerning the genre of Philippians 2:6–11, long considered a poetic ‘Christ-hymn’, but more recently classified under a number of alternative genre headings: exalted prose, encomion, epainos, early Christian confession, didactic poem, and prose hymn. The study examines the text in the light of ancient rhetorical theory of hymnos and notes the essential features of the genre identified by ancient rhetoricians on a descriptive basis. The study then shows the presence of these same features in Philippians 2:6–11, albeit with one key innovation: conventional, Greco-Roman notions of honour and shame that typically shaped the hymnos genre are turned on their end, with the result that the god of the ‘Christ-Hymn’ is praised for taking up conventionally shameful stations in humility and service to others.
This article builds on recent criticism of form-critical approaches to so-called hymnic material in Paul’s letters, in an effort to reset the default interpretive stance with respect to Phil. 2.6-11 and Col. 1.15-20. Beginning with a review and critique of previous form-critical criteria, the authors then survey ancient deﬁnitions of ὕμνος to demonstrate that the oft posited ‘Christ-hymns’ do not fulﬁll these deﬁnitions. In an effort to broaden out the discussion, the reception of these passages in early Christian writings is surveyed, showing that it provides no positive support for identifying these passages as ‘hymns’. The authors conclude that, given the lack of constructive evidence, scholars should reconsider the enduring and widespread operative assumption that these passages are hymns.
Surely there has to be an SBL session and maybe an JSPL issue dedicated to this topic.