[Phl 1:5 YLT] (5) for your contribution to the good news from the first day till now,
[Phl 1:5 NIV] (5) because of your partnership in the gospel from the first day until now,
[Phl 1:5 NKJV] (5) for your fellowship in the gospel from the first day until now,
[Phl 1:5 MGNT] (5) ἐπὶ τῇ κοινωνίᾳ ὑμῶν εἰς τὸ εὐαγγέλιον ἀπὸ τῆς πρώτης ἡμέρας ἄχρι τοῦ νῦν
I suspect that the preposition εἰς provides a good clue.
Also probably relevant:
[Phl 1:7 NLT] (7) So it is right that I should feel as I do about all of you, for you have a special place in my heart. You share with me the special favor of God, both in my imprisonment and in defending and confirming the truth of the Good News.
[Phl 1:7 MGNT] (7) καθώς ἐστιν δίκαιον ἐμοὶ τοῦτο φρονεῖν ὑπὲρ πάντων ὑμῶν διὰ τὸ ἔχειν με ἐν τῇ καρδίᾳ ὑμᾶς ἔν τε τοῖς δεσμοῖς μου καὶ ἐν τῇ ἀπολογίᾳ καὶ βεβαιώσει τοῦ εὐαγγελίου συγκοινωνούς μου τῆς χάριτος πάντας ὑμᾶς ὄντας