im.kayac.comは最高に便利。

$username = 'testuser';
$password = 'testpass';

$data = array(
    "message" => "ほげほえ",
    "password" => $password,
);

$data['sig'] = sha1($data['message'] . $data['password']);
unset($data['password']);

$data = http_build_query($data, "", "&");

//header
$header = array(
    "Content-Type: application/x-www-form-urlencoded",
    "Content-Length: ".strlen($data)
);

$context = array(
    "http" => array(
        "method"  => "POST",
        "header"  => implode("\r\n", $header),
        "content" => $data
    )
);

$url = "http://im.kayac.com/api/post/{$username}";
echo file_get_contents($url, false, stream_context_create($context));

なくしそうだったのでここにメモしておく。