The reCAPTCHA wasn't entered correctly. Go back and try it again.(reCAPTCHA said: incorrect-captcha-sol)

<?php
if (isset($_POST['submit'])){
 require_once('recaptchalib.php');
 $privatekey = "XXXXXXXXXXXXXX";
$resp = recaptcha_check_answer ($privatekey,
                            $_SERVER["REMOTE_ADDR"],
                            $_POST["recaptcha_challenge_field"],
                            $_POST["recaptcha_response_field"]);

if (!$resp->is_valid) {
// What happens when the CAPTCHA was entered incorrectly
die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." .
     "(reCAPTCHA said: " . $resp->error . ")");
 } else {
// Your code here to handle a successful verification
  }
}
?>