Paul explains how he will be “glad and rejoice” over the Philippians even if the price of their faith had to be paid for with his own life. It was well worth it! It would be nothing more than a libation, or a drink offering, that 
was poured out over the main burnt offering. According to Exodus the drink offering was wine. Not the sour vinegar wine that Jesus was offered on the cross, but a robust healthy wine. If you did a study of the Old Testament you’d see how this high quality wine was the symbol of joy. I like the thought of the symbolic joy of the drink offering and Paul’s claim that he was glad to offer himself for the cause of Christ to the Philippians to produce their faith and growth in Christ-likeness.
Up to now most of the references to Joy have been about how Paul, in spite of all his struggles, was still able to maintain his own joy. It’s mostly about Paul’s own joy. But then after expressing his own great joy again in verse 17, Paul goes on in Philippians 2:18 to address their joy. He writes, “Likewise you also should be glad and rejoice with me.” Paul literally and liberally invites them to join his double-dose of joy with a double-dip of their own. If it was a song the “joy note” would show up like the chorus. We see it four times in the space of just these two verses (Philippians 2:17-18).
This is an imperative. We see these constructions as commands. So Hughes says, “Also, this first imperative was totally meshed with Paul’s joy and was a command to rejoice in the midst of suffering. What we have here is a partaking of the fellowship of the gospel at its deepest level (cf. 1:5, 7)—a fellowship rooted in the three-way bond of Paul, Christ, and the Philippians.” Suffering is a part of every life. Christ experienced what must have been the most profound physical, emotional and spiritual suffering of anyone. Paul rejoiced to share in the suffering of Christ. He didn’t go looking for the suffering. Christ did not do that either. But when it came, he made it an offering of joy. Christ suffered as the “whole burnt offering” for the sins of the world. Paul suffered as a drink offering added to the blood of Christ in order to advance the mission of Christ. Our suffering should be viewed in a similar way. In the midst of our sorrow, pain, loss, grief, and misery, let us lift up our eyes to God and offer it all as a second libation (drink offering) with joy in knowing we too are contributing to the joy and faith of others that will follow.