The GPL FAQ "Does the GPL require that source code of modified versions be posted to the public?" contains this statement (emphasis by me)
The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.
But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL.
Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.
My question is, when is the condition
release to the public met / fullfilled?
Let us assume that a website is using a modified PHP-CMS that is licensed under the gpl. Is the fact, that the website which can be reached over the internet already enough to speak of a release of the server side software?
What if only logged in users can use the cms?