tmp.php

<?php

function hmac($key, $data, $hash = 'md5', $blocksize = 64) {
  if (strlen($key)>$blocksize) {
    $key = pack('H*', $hash($key));
  }
  $key  = str_pad($key, $blocksize, chr(0));
  $ipad = str_repeat(chr(0x36), $blocksize);
  $opad = str_repeat(chr(0x5c), $blocksize);
  return $hash(($key^$opad) . pack('H*', $hash(($key^$ipad) . $data)));
}

echo hmac('hi', 'blah');

?>