melewati recaptcha sederhana

<?php
if(isset($_POST['g-recaptcha-response'])){

    $cookie="cookie.txt";
    if(!file_exists($cookie)){
            $fh = fopen($cookie, "w");
            fwrite($fh, "");
            fclose($fh);
    }
        $captcha=$_POST['g-recaptcha-response'];

        $postdata = "username=xxx&password=xxx&g-recaptcha-response=$captcha";
        $ch = curl_init();
        curl_setopt ($ch, CURLOPT_URL, "REMOTE URL LOGIN");
        curl_setopt ($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt ($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6");
        curl_setopt ($ch, CURLOPT_TIMEOUT, 60);
        curl_setopt ($ch, CURLOPT_FOLLOWLOCATION, 1);
        curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt ($ch, CURLOPT_COOKIE,1);
        curl_setopt ($ch, CURLOPT_COOKIEJAR,$cookie);
        curl_setopt ($ch, CURLOPT_COOKIEFILE,$cookie);

        curl_setopt ($ch, CURLOPT_POSTFIELDS, $postdata);
        curl_setopt ($ch, CURLOPT_POST, 1);

        $result = curl_exec($ch);

        echo $result;
}else{
?>
    <script src="//www.google.com/recaptcha/api.js" type="text/javascript"></script>
    <form method="post" action="" name="signup-form">
        <center><div class="g-recaptcha" data-theme="light" data-sitekey="SITEKEYREMOTELOGIN"></div></center>
        <button type="submit" name="submit">SUBMIT</button>
    </form>
<?php
}
?>