Even Christians shipwreck themselves on the rocks of discontent during stormy circumstances. Paul's statement is made in anticipation of what the Philippians Christians might think about his situation. In the previous verses, he has told them how thankful he is that their concern for him has been renewed. Not that they weren't always concerned, but they lacked opportunity to show it. In other words, Paul wasn't hovering over the mailbox, wringing his hands.
Christians hear much but learn little.
He learned that having everything did not enhance him and that having nothing did not diminish him. Unlike so many of us his contentment wasn't conditioned by circumstances. He had learned to live independent of earthly conditions. Simply stated, Paul had learned to live from the inside out. What was in him was greater than anything outside him.