1 unicode.inc mime_header_encode($string)

Encodes MIME/HTTP header values that contain incorrectly encoded characters.

For example, mime_header_encode('tést.txt') returns "=?UTF-8?B?dMOpc3QudHh0?=".

See http://www.rfc-editor.org/rfc/rfc2047.txt for more information.


  • Only encode strings that contain non-ASCII characters.
  • We progressively cut-off a chunk with truncate_utf8(). This is to ensure each chunk starts and ends on a character boundary.
  • Using \n as the chunk separator may cause problems on some systems and may have to be changed to \r\n or \r.


$string: The header to encode.

Return value

string: The mime-encoded header.

See also



core/includes/unicode.inc, line 390
Provides Unicode-related conversions and operations.


function mime_header_encode($string) {
  if (preg_match('/[^\x20-\x7E]/', $string)) {
    $chunk_size = 47; // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);
    $len = strlen($string);
    $output = '';
    while ($len > 0) {
      $chunk = backdrop_truncate_bytes($string, $chunk_size);
      $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\n";
      $c = strlen($chunk);
      $string = substr($string, $c);
      $len -= $c;
    return trim($output);
  return $string;