SPBAS (SolidPHP Business Automation Suite) is a good software package that features a web store, client area, and licensing for php scripts (it even comes with a php class that you can plug right into your scripts!). Best of all, it's free for your first 5 clients, after which you will need to pay to continue accessing the admin area (although new clients will still be able to purchase your products).
It also integrates nicely with the ionCube PHP Encoder, which is a reasonably cheap encoder that protects the source code of your PHP scripts. If you plan on using SPBAS's licensing component, you would only need the Basic encoder (aka the cheapest one), so there's an extra benefit in that.
SPBAS reduces the traffic of callbacks by allowing you to configure how frequently it calls back. For example, you could have it set so that it dials home every two weeks to grab a temporary license file from your server that expires in 2 weeks, even if the main license doesn't expire or has a much longer expiration date.
Finally, SPBAS is currently running a deal in conjunction with ionCube where you can get a discount on the ionCube Basic encoder when purchasing it in conjunction with SPBAS.
You can read more about SPBAS at
http://www.spbas.com and ionCube at
http://www.ioncube.com
Note: I am recommending the above solution because that is what I am using for my scripts, and it seems to be working out well enough. There are obviously tons of other solutions and competing products that you may wish to check out as well before deciding on the above in order to get a better idea of what's out there and what solutions fit your needs the most.