mb_encode_mimeheader() encodes a given string str by the MIME header encoding scheme. Returns a converted version of the string represented in ASCII.
charset specifies the name of the character set in which str is represented in. The default value is determined by the current NLS setting (mbstring.language).
transfer-encoding specifies the scheme of MIME encoding. It should be either "B" (Base64) or "Q" (Quoted-Printable). Falls back to "B" if not given.
linefeed specifies the EOL (end-of-line) marker with which mb_encode_mime_header() performs line-folding (a RFC term, the act of breaking a line longer than a certain length into multiple lines. The length is currently hard-coded to 74 characters). Falls back to "\r\n" (CRLF) if not given.
Nota: This function isn't designed to break lines at higher-level contextual break points (word boundaries, etc.). This behaviour may clutter up the original string with unexpected spaces.
See also mb_decode_mimeheader().