For me the biggest challenge is that at the company that I work for is too strict in trying out "unknown" technologies. So I can only learn in my free time. I don't really have any real project ideas in which I can try out my knowledge, so I forget the most of what I learned a few weeks ago. I think it's hard to change, when your job ties you to php. And in my country (Hungary) anything other than php, or java for web development is pretty unknown.