Staying Focused for Long Periods of Time

I seem to have a problem staying focused on something for long periods of time. When I say "long", I'm talking about doing things for about a week or two straight, after which I just lose interest and start doing something else. You may or may not recall that just a week or two ago, I was trying to learn some more PHP. I was doing this for a good week straight, when something just slipped and I ended up getting involved with something else instead. Now I can't stop doing something else (playing Team Fortress 2), and PHP is right at the back of my mind until I next decide to come back to it again.

Is this normal? I mean, is it normal to lose a little interest in something after a week? I know I'll come back to it, but it's a shame I stopped so early.

Also, will this hinder my learning or is it good to get a break from doing something? Will this actually help me to remember information?