It was autonomous during both the Greek and Roman Empires.
Livios.org even says that in the Greek era for example, people were registered as Philippians and not Macedonians.* (so the reference to the “Book of Life” is both spiritual and has socio-political significance as well)
They were immune from paying tribute.
They had the same rights and were considerd as Roman citizens as if they were living in Italy.
Sources: Halley’s Bible Handbook (2000 edition) pp. 812 *Livius.org: http://www.livius.org/phi-php/philippi/philippi.html