some books, php.net, phpbuilder.com and .chm manual is the best teachers 🙂 and the big thing is an expirience. my first works i made only for exp, even not for resume (first works was a terrible works 🙂)
so you learn more and more, work hard, and one day the feeling comes - you're the real professional. you know enough, to help another people and even write a book.
and there is th Main Thing - you haven't to stop learning. beeng a profi is like a swimming against the stream - as soon as you relax, you start moving backward.
so... it's only up to you to decide, how professional you can be.
sorry, if my english... not professional enough 😉