I installed MediaWiki 1.27.0 on Debian Jessie with PHP 5.6.22 last week and finally got around to start editing it. However, the account I've created gets a login failure, namely: "There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Go back to the previous page, reload that page and then try again."
The PHP session directory exists and should be writable by the PHP process and webserver from what I can see. (I found this discussion Topic:Pjby0sdeg3e60rfy which isn't very edifying, and also six years old, but it's still the least bad source of possible solutions I could find -- everything else is about as old and has less useful information).
Let me know if you need any more information, and I'll do my best to provide it.