function sha1_hmac($key,$data,$blockSize=64,$opad=0x5c,$ipad=0x36) {

    // Keys longer than blocksize are shortened
    if (strlen($key) > $blockSize) {
        $key = sha1($key,true);
    }

    // Keys shorter than blocksize are right, zero-padded (concatenated)
    $key       = str_pad($key,$blockSize,chr(0x00),STR_PAD_RIGHT);
    $o_key_pad = $i_key_pad = '';

    for($i = 0;$i < $blockSize;$i++) {
        $o_key_pad .= chr(ord(substr($key,$i,1)) ^ $opad);
        $i_key_pad .= chr(ord(substr($key,$i,1)) ^ $ipad);
    }

    return sha1($o_key_pad.sha1($i_key_pad.$data,true),true);
}