Scholars used to assume Philemon was written from Rome because Paul is writing from prison, but there is no necessity for a Roman locale, and Edgar Goodspeed persuasively argued years ago that Ephesus is much more plausible. Paul writes the letter to entreat his friend Philemon in Laodicea to accept and forgive the runaway slave Onesimus who had come under Paul’s influence while Paul was in prison. Ephesus is the nearest big city to Laodicia, a far more likely place for Paul to have encountered Onesimus than Rome. See Goodspeed, An Introduction to the New Testament (Chicago: Univ. of Chicago Press, 1937), pp. 109–124.