Manual:$wgImgAuthPublicTest/en

Details
This configuration variable is only applicable if you are using Image Authorization.

It forces a preliminary check to see if the wiki is a full public wiki ( $wgGroupPermissions['*']['read'] = true; ) because you really don't need to use Image Authorization if you have a public wiki. MediaWiki will warn you about an incorrect configuration when this is set.

However, there are cases where the default is for full public access, but access restrictions apply (such as only approved users can edit, ...). If that is the case, you will need to set this to false in order for the configuration check to be bypassed.

This functionality was brought forward from the img_auth.php prior to 1.16.