to is mail addresses send to. Multiple recipients can be specified by putting a comma between each address in to. This parameter is not automatically encoded.
subject is subject of mail.
message is mail message.
additional_headers is inserted at the end of the header. This is typically used to add extra headers. Multiple extra headers are separated with a newline ("\n").
Content-Transfer-Encoding headers can be redefined as
of PHP 5.0.0. In PHP 4, values defined by
mb_language() are always used.
additional_parameter is a MTA command line parameter. It is useful when setting the correct Return-Path header when using sendmail.