There is not a standard belief among various bible scholars about what this means, but I suppose in general it could be said to the agreement of most that the prophecy is referring to a remnant of the Philistines, which Ekron was a city of, who would become proselytes of Judaism as a result of the destruction that was to fall on them. Possibly this destruction occurred when Alexander the Great helped the Jews and at the same time destroyed the neighbouring countries.
If as a result a significant group of survivors became God fearing attendees of the synagogue I do not know. If so, the Christian view would be that this would be a preliminary fulfillment only. As the verse suggests some in superior roles and some like Ekron in an inferior role, as some Jebusites at the time of David were assimilated into Israel as inferiors, we get the prophecy that all in all the difference between the remnant of both Jew and Gentile will be removed. Furthermore, as the next Chapter launches into the subject of Messiah, it seems clear the conversion of this remnant of the Philistines is symbolic for all the nations being judged. It means those gentile converts to the King of the Jews, the Lord Jehovah in Christ.
So when did the Philistines join Israel? Well the Christian view is that they still are and they begun in great numbers under the Apostle Paul. They have already added up to the number of stars in the sky, as Abraham's descendants were promised to be according to faith in his promised seed.