vi /etc/phpmyadmin/config.inc.php
You may need to use sudo; vim will warn you if you try editing a read-only file.
Edit an existing LoginCookieValidity setting if any, or (as in our case) add the following to the bottom of the file.
(Note that in vim you can search with /LoginCookieValidity and, if finding nothing, go to the end of the file with :999 -- actually that's go to line 999 which is just as good in this case, there's a command to do it for real too). Then you can press i to insert text:
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