In my opinion it is highly unlikely that Phil. 2:5-11 is a pre-Pauline hymn. I could go on at some length. Bibliography on the passage is listed in W.G. Ku"mmel, Introduction to the New Testament, rev. ed. London: SCM, 1975, pp. 334-35, n. 38. The principal English work is R.P. Martin, Carmen Christi, 1967.