I am working on a webapp (no content by me, all user driven) and want to license it with the GPL 3. I do plan on having a tarball with the php, html, css, javascript, etc as well as an installer to set up a mysql database for it and some other small stuff so someone else can host a personalized one if they want (like phpbb). The GPL requires that the user be allowed to modify and run the modified software. I obviously cannot allow that on my server, but am not sure if that stopping the user like that violates the terms of the GPL. Also, I do plan on an easy, readable (probably csv) export/import option so the user can move their data around if they want.Is that enough to placate the GPL? 
GPL 3 and webapps
php;web applications
Offering the source code for download is enough. You're not required to give users the ability to modify the source code in your hosted version.This has always been the case. The 'freedom to modify and run' does not signify a freedom to modify and run source code on a machine the user neither owns nor has admin access to.There's some clarification of the rules for installed software in the GPLv3 FAQ:Companies distributing devices that  include software under GPLv3 are at  most required to provide the source  and Installation Information for the  software...In short, give users access to the source code and instructions to install it, and that's enough to satisfy the licence. (e.g. Like Automattic does with WordPress.)