Recaptcha always failing on the second time

Question

I'm loading Recaptcha using this:

<script src="//www.google.com/recaptcha/api/challenge?k=key"></script>

and using AJax and PHP to validate the response:

$ch=curl_init('http://www.google.com/recaptcha/api/verify');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, array(
    'privatekey'=>'key',
    'remoteip'=>$_SERVER['REMOTE_ADDR'],
    'challenge'=>$_POST['recaptcha_challenge_field'],
    'response'=>$_POST['recaptcha_response_field']
));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response=@curl_exec($ch);

if(substr($response,0,4)!='true')
    die('The verification code is incorrect.');

For some reason, if I enter the Captcha incorrectly, then re-enter it correctly, the verification always returns "false". How do I allow the user the resubmit the Recaptcha response (without reloading the image) if they entered it incorrectly the first time?

Answer 1

You can't. The user cannot be given a second attempt at a reCaptcha challenge — if they get one wrong, they will need to attempt a different one the next time. For a web form, this should typically be implemented by redisplaying the form with an error message if the reCaptcha challenge fails, rather than forcing the user to use the "Back" button.