in_array function Problem

I’d assume that $specific_admin_usernames is an array– so there’s no need for this line $specific_admin_usernames_imploded = implode($master_admin_usernames);

Also your if statement is a little backwards. PHP’s in_array( $needle, $haystack) doesn’t need the $current_logged_in_user !== part. Try:

if ( in_array($current_logged_in_user, $master_admin_usernames)  ){
    // this runs when the current user is IN the master list 
}