I know this is old but I am seeing it today, and have 2 cents to add.

mb_strstr isn't called by anyone, it's an internal function to strstr.

In this admin/includes/init_includes/init_file_db_names.php file line 20, the $_SERVER['HTTPS'] and $_SERVER['HTTP_X_FORWARDED_BY'] are empty, hence the haystack is empty to the internal strstr function mb_strstr.

$request_type = (strtolower($_SERVER['HTTPS']) == 'on' || $_SERVER['HTTPS'] == '1' || strstr(strtoupper($_SERVER['HTTP_X_FORWARDED_BY']),'SSL') || strstr(strtoupper($_SERVER['HTTP_X_FORWARDED_HOST']),'SSL')) ? 'SSL' : 'NONSSL';



This is a mis-configuration on the hosting side that is not passing these two variables.