Undefined index: recaptcha_challenge_field; recaptcha_response_field

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PHP

Hi, I'm trying to implement a reCAPTCHA to a register user page in
PHP, but I continue to get a warning, and my users don't get
registered in the database. Before trying to add reCAPTCHA the
registration worked as it should.

Warning 1: Undefined index: recaptcha_challenge_field (line that gives
this warning is $_POST["recaptcha_challenge_field"],)

Warning 2: Undefined index: recaptcha_response_field (line that gives
this warning is $_POST["recaptcha_response_field"]);)

------------------ My Code ---------------------

<?php <br mode="hold" /??>        // reCAPTCHA stuff 
        require_once('recaptchalib.php'); 
        $publickey  = "..."; 
        $privatekey = "..."; 
        // Tripplechecked that the keys are correct! 

        $resp         = null; 
        $error         = null; 
        // end reCAPTCHA stuff 

        if($_POST) 
        { 
                $username        = $_POST['username']; 
                $password         = $_POST['password']; 
                $confirm                 = $_POST['confirm']; 
                $email                 = $_POST['email']; 

                // Some checks to see if data was entered in the form 
                if($username == "") 
                { 
                        // Logic for checking data 
                } 
                else 
                { 
                        // Connect to database and check for already existing user names / 
e-mails 
                        // If none found, proceed with the below code 

                        // reCAPTCHA stuff 
                        $resp = recaptcha_check_answer ($privatekey, 
                                $_SERVER["REMOTE_ADDR"], 
                                $_POST["recaptcha_challenge_field"], 
                                $_POST["recaptcha_response_field"]); 
                        if(!$resp->is_valid) 
                        { 
                                echo $resp->error; 
                        }// end reCAPTCHA stuff 
                        else 
                        { 
                                // Register new user in database 
                        } 
                } 
        } 
?> 

PLZ HELP TO RESOLVE THIS PROBLEM