#6218 - applying hotfix for `iconv.internal_encoding` deprecation

Fixes #6218

See https://wiki.php.net/rfc/default_encoding

 > PHP 5.6 and master, introduce new php.ini setting. Old iconv.*/mbstring.* php.ini parameters will be removed for master PHP6. Use of iconv.*/mbstring.* php.ini parameters raise E_DEPRECATED for 5.6 and up.