Looking for a good User Agreement/Proxy Server/HotSpot Software

I’d like guests on our LAN to digitally sign a use agreeement in order to access the Internet. I’d like to have this work somewhat like how hotel internet access works (HotSpot Software). You get an IP address, start your browser, and no matter what your homepage is set to, an agreement page pops up which has to be agreed to for internet access. I’d rather use Linux/PHP/Apache/MySQL if possible rather than anything needing to run on Windows. Any advice would be greatly appreciated. Thanks!