For your fellowship in the gospel from the first day until now;
Note 4 at Php 1:5: Why is it that Paul had such joy and thanksgiving every time he thought of the Philippians? He gave the answer here. It was because of their "fellowship in the gospel." The word "fellowship" was translated from the Greek word "KOINONIA," and this Greek word means "partnership, i.e. (literally) participation" (Strong's Concordance). This word was translated "contribution" in Ro 15:26 and "distribution" in 2Co 9:13. Both of these words were dealing with finances.
Paul very clearly stated, later in this same letter (Php 4:15-16), that the Philippians were the only church that supported him in the beginning of his ministry. One of the reasons Paul had such fond thoughts about the Philippians was that they had sacrificially supported him with their finances. Certainly, there were other things involved, but finances were one of the important ways the Philippians showed their love and support for Paul.
Note 5 at Php 1:5: As explained in note 4 at this verse, Paul was referring to the Philippians' support for him, including financial contributions. Here, he said that this support was "from the first day until now." Certainly, one of the things that caused Paul to rejoice every time he thought of the Philippians was their faithfulness over a period of time.
It is a blessing to have people express their love to any degree, but, sad to say, this type of fellowship doesn't typically last very long. Partnerships are hard to maintain. Many things work against partnerships, and most don't last long. It is a real joy to ministers to have people who were ministered to many years ago remember them and still support the work they are doing.