Decodes modified UTF-7 text into ISO-8859-1 string.
Returns a string that is encoded in ISO-8859-1 and consists of the same
sequence of characters in text, or
if text contains invalid modified UTF-7 sequence
or text contains a character that is not part of
ISO-8859-1 character set.
This function is needed to decode mailbox names that contain certain characters which are not in range of printable ASCII characters.
See also: imap_utf7_encode().