Message-ID generator in PHP for mail headers.

message-id.php

<?php

/**
 * @see http://www.jwz.org/doc/mid.html
 */
function generateMessageID()
{
  return sprintf(
    "<%s.%s@%s>",
    base_convert(microtime(), 10, 36),
    base_convert(bin2hex(openssl_random_pseudo_bytes(8)), 16, 36),
    $_SERVER['SERVER_NAME']
  );
}