The basic elements of this chapter thus far are:
Imprisoned at Ephesus?
Ephesus at the End
And now part III:
Living Worthy of the Gospel
The key verse of Philippians would seem to be 1:27. After Paul has greeted the Philippians (1:1-2), after he has thanked God for them and rehearsed some of his current situation (1:3-26), he begins the body of the letter with an admonition to "live your life in a manner worthy of the gospel of Christ" (1:27). Most of the rest of the letter, we might argue, plays out this general instruction.
Perhaps the main goal Paul has in mind is for the Philippian church to be unified and loving toward one another, both in the face of opposition and in terms of tensions among themselves. Just after he tells them to live worthy lives, he says he wants to hear that they "are standing firm in one spirit, striving side by side with one mind for the faith of the gospel" (1:27). He wants them to be "of the same mind, having the same love, being in full accord and of one mind" (2:2). He wants them to look out not only for themselves but for others as well (2:4) and not to operate out of selfish ambition (2:3).
Near the end of the letter, he specifically calls out two women who worked together with him when he was at Philippi: Euodia and Syntyche (4:2). As he earlier encouraged the entire assembly, he urges them in particular to "be of the same mind in the Lord." These two had struggled beside Paul in the work of the gospel, along with someone named Clement and, perhaps, Epaphroditus.  But somewhere along the way they had come into conflict. As he had with the Corinthians, he urges the Philippians to heal the disunity among them.
The most majestic part of Philippians is the so called Philippian hymn of 2:6-11. Paul uses the example of Christ's selfless suffering for others as a model for how the Philippians should behave toward one another (2:5). We cannot be certain whether Christians somewhere actually sung these words. The rhythm of the words is not entirely clear and the lines are not all the same in length. Further, most scholars think that wherever the poem came from, Paul has interrupted it at some points with commentary. Would a group used to singing such words not have been irritated at having the flow of a hymn interrupted? 
Nevertheless, the words do have a rhythm to them, and we do know that the early Christians composed and used hymns in their worship (e.g., 1 Cor. 14:26). While we cannot know for certain, a good suggestion is that the hymn falls into three parts of four lines each, with Paul expanding on a few points. The places where we think Paul might have added to the hymn are in parentheses. The translation is our own:
Although he existed in the form of God,
He considered equality with God not something to exploit,
But he emptied himself,
Having taken the form of a servant.
Having become in the likeness of mortals,
And having been found in shape as a mortal,
He humbled himself,
Having become obedient to the point of death (even death on a cross).
Therefore, God even super-exalted him,
And graced on him the Name above every name,
That (at the name of Jesus) every knee should bow (whether in heaven or on earth or under the earth) and tongue confess,
That Jesus Christ is Lord (to the glory of God the Father).
This reconstruction, as all of them, is not without its problems, leading some simply to abandon the notion that we even have a clear poem here. Many are content to suggest that Paul has simply waxed somewhat poetic on the spot. The rhythm of our last stanza above in particular is difficult and different from the first two, which actually have somewhat of a parallel structure. Nevertheless, the poem seems to have enough rhythm with apparent interruptions to suggest that Paul is drawing from somewhere else here.
Several features of the hymn are quite remarkable. Most take the first stanza to be about Jesus' pre-existence before he assumed human form.  Yet the most remarkable part of the hymn is in the third stanza, where God gives to Jesus the "Name above every name." The "Name above every name" can hardly be anything but a reference to Yahweh, the very name of God. Yahweh was translated into Greek as "Lord," and so the Name that God bestowed on Jesus at the point of his resurrection was surely "Lord," not Jesus. The hymn thus sees Jesus receiving the very Name of God as God raises Jesus from the dead and super-exalts him to His right hand.
Paul uses this hymn about Christ to show the Philippians both the attitude they should have and, just perhaps, what the ultimate benefit of such an attitude is. First, Jesus was none other than the Son of God, a status of royalty and kingship that none of them certainly had. Yet even with such a high status, Jesus did not consider his divine status as something to take advantage of. Rather he assumed the status of a servant. And even once he had assumed the shape of a mortal, even though he looked like any other human being, he humbled himself even further. He endured the most shameful form of capital punishment in use at the time: crucifixion.
It was to these lengths of service and sacrifice that Paul called the Philippians to embrace in the interests of each other. And of course as a result God took Christ even higher perhaps than he had been before. If he had been in the form of God before, now he was super-exalted with the very name of Yahweh. So also, perhaps Paul means to imply, the Philippians will eventually be exalted by God if they will be servants now.
The final chapter of Philippians is filled with other items of "worthy living" Paul advises as they live as citizens of heaven (1:27; 3:20).  Their true citizenship is not on earth, tied up with any earthly city or empire. Paul tells them not be anxious about anything but to rely on God (4:6). He tells them to let others see how gentle they are (4:5). He encourages them to think about noble and uplifting things (4:8). Throughout he has a mellow spirit. His ordeal has humbled him, and we see this same spirit in 2 Corinthians, which we date not long after Paul wrote Philippians.
 Paul addresses at this point "my loyal companion" or "my loyal yokefellow." Some have suggested the word yokefellow is actually this man's name, Syzygus. But we think Paul is likely referring to Epaphroditus, who probably was the one who delivered the letter of Philippians back to Philippi.
 Or so has argued N. T. Wright. One of his early books was a collection of articles titled The Climax of the Covenant (***), which includes articles on key texts about Christ like these verses from Philippians. Whether you agree with him or not, we put this book among the top fifty or so books one should read to master Paul's writings.
 With some notable exceptions. For example, James D. G. Dunn is perhaps the best known proponent of the idea that the first line is actually about Christ being the second Adam who undid the Fall (Christology in the Making ***). Adam was also in the "image of God," but he did grasp at equality with God. Jesus, in the same situation, did not grasp in that way. Dunn thus did not see any indication of Christ's pre-existence in the hymn, since it was as a human on earth that Jesus did not take advantage of being in the image of God. Most, however, have not followed Dunn's interpretation. For example, it is not clear that "form of God" means the same thing as "image of God."
However, a strong alternative to the majority position is the possibility that form of God relates to Jesus having the status of God, that is, of being God's Son. This view was most notably held by Eduard Schweizer (***). The idea that form of God has to do with status is supported by the fact that it is parallel to the phrase "form of a servant." The stanza thus comes to say that while Jesus had the status of God, he did not take advantage of that status, but instead took on the status of a servant. On this reading as well, it would not be entirely clear that Paul was referring to Jesus in some pre-existent state.
 N. T. Wright is at some pains to make it clear that this expression does not mean Paul is saying that we will go to heaven or spend eternity in heaven, only that heaven is where our empire is centered (***).