An alternative to the insecure login plugin referenced on Adminer's official site. See the integration guide for plugins then add this plugin, as below, to grant access without revealing the authentication to the user.
new AdminerLoginExternal([
# Normal host & port, or socket.
'server' => $server,
# Prefill db name for convenience.
'database' => $db,
'username' => $db_user,
'password' => $db_user_pw,
# Omit for MySQL (the default), specify Adminer driver name otherwise.
'driver' => 'pgsql',
# Boolean of whether the current user is allowed access as the above db
# user. Up to you how to implement that, but you should be checking this on
# *every request* to Adminer ensure it's false when you can't verify them.
# Don't use literal "true" unless you're using this on a private development
# machine and PHP is only litening on a loopback address.
'authenticated' => $logged_in,
# Optionally override default title 'Adminer'.
'app_name' => 'MyCo DB admin',
# Optionally require an extra step to "log in" to Adminer. More secure and
# maybe necessary if using other plugins that modify the login form,
# e.g.: offer database selection.
'manual_login' => true,
# Optional HTML message to show when user's external authentication expired
# whilst logged into Adminer.
'expired_html' => <<<'EOHTML'
<p>Your MyCo authentication expired.</p>
EOHTML,
# Optional HTML message to show user they are not authenticated.
'failure_html' => <<<'EOHTML'
<p>You need to <a href="/path/to/3p/log-in/">log in</a>.</p>
EOHTML,
])