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http://releasenotes.docs.salesforce.com/en-us/spring17/release-notes/rn_sales_web_to_lead_recaptcha.htm

http://malebolgia.shadowsonawall.net/code-programming/captcha-with-salesforce.html

https://github.com/tutsplus/how-to-integrate-google-no-captcha-recaptcha-on-your-website/blob/master/recaptchalib.php

<?php

/**
 * A ReCaptchaResponse is returned from checkAnswer().
 */
class ReCaptchaResponse
{
    public $success;
    public $errorCodes;
}
class ReCaptcha
{
    private static $_signupUrl = "https://www.google.com/recaptcha/admin";
    private static $_siteVerifyUrl =
        "https://www.google.com/recaptcha/api/siteverify?";
    private $_secret;
    private static $_version = "php_1.0";
    /**
     * Constructor.
     *
     * @param string $secret shared secret between site and ReCAPTCHA server.
     */
    function ReCaptcha($secret)
    {
        if ($secret == null || $secret == "") {
            die("To use reCAPTCHA you must get an API key from <a href='"
                . self::$_signupUrl . "'>" . self::$_signupUrl . "</a>");
        }
        $this->_secret=$secret;
    }
    /**
     * Encodes the given data into a query string format.
     *
     * @param array $data array of string elements to be encoded.
     *
     * @return string - encoded request.
     */
    private function _encodeQS($data)
    {
        $req = "";
        foreach ($data as $key => $value) {
            $req .= $key . '=' . urlencode(stripslashes($value)) . '&';
        }
        // Cut the last '&'
        $req=substr($req, 0, strlen($req)-1);
        return $req;
    }
    /**
     * Submits an HTTP GET to a reCAPTCHA server.
     *
     * @param string $path url path to recaptcha server.
     * @param array  $data array of parameters to be sent.
     *
     * @return array response
     */
    private function _submitHTTPGet($path, $data)
    {
        $req = $this->_encodeQS($data);
        $response = file_get_contents($path . $req);
        return $response;
    }
    /**
     * Calls the reCAPTCHA siteverify API to verify whether the user passes
     * CAPTCHA test.
     *
     * @param string $remoteIp   IP address of end user.
     * @param string $response   response string from recaptcha verification.
     *
     * @return ReCaptchaResponse
     */
    public function verifyResponse($remoteIp, $response)
    {
        // Discard empty solution submissions
        if ($response == null || strlen($response) == 0) {
            $recaptchaResponse = new ReCaptchaResponse();
            $recaptchaResponse->success = false;
            $recaptchaResponse->errorCodes = 'missing-input';
            return $recaptchaResponse;
        }
        $getResponse = $this->_submitHttpGet(
            self::$_siteVerifyUrl,
            array (
                'secret' => $this->_secret,
                'remoteip' => $remoteIp,
                'v' => self::$_version,
                'response' => $response
            )
        );
        $answers = json_decode($getResponse, true);
        $recaptchaResponse = new ReCaptchaResponse();
        if (trim($answers ['success']) == true) {
            $recaptchaResponse->success = true;
        } else {
            $recaptchaResponse->success = false;
            $recaptchaResponse->errorCodes = $answers [error-codes];
        }
        return $recaptchaResponse;
    }
}
?>