Philemon 19  “…albeit I do not say to thee how thou owest unto me even thine own self besides.”
Very strange letter.  Paul says he trusts Philemon to do the right thing, the Christ-like thing, but he reminds him of all the other things that Paul could use as leverage.  Makes you wonder what was going on behind the scene.

Philemon 23-24  “…Epaphras…Marcus, Aristarchus, Demas, Lucas…”
These guys are also mentioned at the end of Colossians.  Verses in Colossians tell that Paul was sending Tychicus and Onesimus to Colossae.  Here, he is sending him back to Philemon.  The context gives evidence that the letters were written and sent at the same time.  Background on the two books supports this.

Please feel free to comment or post questions.  Thanks for reading!