PHP Curl と Cookie を使用して複数のユーザー認証を処理する方法は?

function fetch($url, $z = null) {
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_AUTOREFERER, true);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt($ch, CURLOPT_POST, isset($z['post']));

    if (isset($z['post'])) {
        curl_setopt($ch, CURLOPT_POSTFIELDS, $z['post']);
    }
    if (isset($z['refer'])) {
        curl_setopt($ch, CURLOPT_REFERER, $z['refer']);
    }

    curl_setopt($ch, CURLOPT_USERAGENT, $useragent);
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, (isset($z['timeout']) ? $z['timeout'] : 5));
    curl_setopt($ch, CURLOPT_COOKIEJAR, $z['cookiefile']);
    curl_setopt($ch, CURLOPT_COOKIEFILE, $z['cookiefile']);

    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}