Teaching is one of the best way to learn more about a subject. It does not matter how you choose to teach it. It could be presenting to a group, or one-on-one mentoring. When you prepare, you will come across new materials. Or you will be asked a question you did not expect. But you can be sure your knowledge will be challenged at some point and you will learn from it.
So go ahead and share your knowledge with someone. A great place to start is at phpmentoring.org.