Help with script licensing?

I hope this is the right area to ask this. I have a wordpress plugin, that I'd like to distribute. I want it to be free (price), and I want to allow people to modify it BUT I'd like the licence to require users who make changes to post them back at the source.

Is there a particular licence out there that allows this, or am I best just to use a GPL or HPL licence?

Thanks for your help.

 

I suggest that you use GPL

 

Thanks, that's what I'm going to do.