When we think of apostles, we often just think of the Apostles. But apostle means anyone who is a messenger for another (literally it is "one who is sent"). Anyone who spreads the Word and announces the love of God is an apostle.
Philip met Don Bosco at a very early age and tied his life to the Salesians, mentored by Bosco himself. Even before his vows he was placed into positions of authority. He worked closely with Don Bosco, learning from him his principles that drove his educational movement.
So what? As Rector Major of the order, armed with that intimate knowledge of the saint, he moved the order from one century to another, guiding the transition of those principles to a "modern" world. He also supported and developed of the Order of Our Lady Help of Christians for women. It was said that he had an empathtic understanding of the problems women faced in the religious life and that is reflected in the planning and rule of the order.
We are always the apostle of another, whether it was Philip of Don Bosco or, ultimately, of Jesus. But we always bring the gift of ourselves to the apostleship. What is great is made greater by our humble dedication to Jesus. Think about how many of the Apostles we know so little about, and yet where would we be without their efforts?
What must you do to have life? Before all else, the first thing you have to do is pray for courage every day to carry the cross the Lord has assigned you. Then let each of you do your own work really well, the work proper to your state, as God wants it, and according to your condition, which means according to the spirit of the Lord and of Don Bosco.
-- From a Letter