The Philippians are fighting the same battle they saw Paul fight earlier in Philippi. Acts 16:11–40 tells of the difficulties Paul encountered in Philippi. In 1 Thessalonians 2:2 he mentions the beatings and insults he endured in Philippi. Paul still suffers for Christ. After all, he is in jail. Thus, Paul encourages them. He and the Philippians fight the same battle together and they suffer together for Christ.
30 engaged in the same conflict that you saw I had and now hear that I still have.