I'm running 2018-04-22a "Greebo", which I upgraded to because I had this bug with an older version, but it continues.
The issue is pretty straightforward. When my PHP version is set to 5.6 my wiki works. With PHP 7.0, 7.1, or 7.2, I get these error messages (even if I'm not trying to log in -- just at the start of doku.php):
Pink background: "User authentication is temporarily unavailable. If this situation persists, please inform your Wiki Admin."
Mauve background: "No ACL setup yet! Denying access to everyone."
Sorry, you don't have enough rights to continue.
You are currently not logged in! Enter your authentication credentials below to log in. You need to have cookies enabled to log in.
I have searched for previous occurrences of this issue, but they all seem to involve acl.auth.php. I presume that if that was the problem, my Wiki wouldn't work with PHP 5.6 either. Indeed, I can provoke a similar set of messages by commenting out the "* @ALL 1" line from acl.auth.php, except that I don't see the pink-background message saying "User authentication is temporarily unavailable. If this situation persists, please inform your Wiki Admin."
I have this in my local.php file:
$conf['authtype'] = 'authmysql';
So I presume the problem is that something about the MySQL setup that I have doesn't like PHP 7.x. Again, flipping back to PHP 5.6 fixes the problem instantly.
Any help would be appreciated.