Even Philippians has its doubters!
Quote: Burton Mack writes (Who Wrote the New Testament?, pp. 144-145):
The letter is actually composed of three letter fragments, accidentally saved as it appears and crudely joined together at some later time by those who collected the letters of Paul in the name of the Pauline school (Phil. 4:10-20; 1:1-3:1; 3:2-4:9). The first two seem to have been written from Ephesus around the time of Paul's imprisonment there (ca. 54-55 C.E.), or five to eight years after Paul first established the congregation in Philippi. . . It is possible that this third letter fragment was not originally addressed to Philippi at all but inserted between the other two letter fragments because of the personal tone.