There is good reason to believe that Paul is using an early Christian hymn in this section of his Letter to the Philippians. Paul may or may not be the original author of the hymn, but that is not very important. Apart from its contents, the very existence of this hymn is evidence that early Christian communities were already firm in their belief in the divinity of Jesus. Though the later great Councils of the Church help to clarify and formulate the doctrine of Christ's divinity, they did not invent that doctrine.
It is also important to realise that these verses are a hymn, not a theological treatise. Hymns are used in liturgy, where the community worships God. The early community which sang this hymn during its liturgy was worshipping Jesus as God. Their liturgy and spirituality absorbed their doctrine, while the doctrine gave true substance to their liturgy.
There is also evidence that, in using this hymn for his Letter, Paul added the words "death on a cross" as the centre, the very centrepiece, of the hymn. As a Pharisee, Paul strongly rejected the notion of the cross. After his conversion, when he firmly believed in Jesus the Messiah, he made the doctrine of the cross central to his preaching.
Lord Jesus, grant us the grace always to live according to Your mind and heart.