I cant able to mix the both feature (mail() + captcha together)

if(isset($_POST['submitregisterbutton'])){

    $captchaenter = $_POST['inputcaptcha'];

if($user_captcha!=$_SESSION['captcha_string'])
    {

    $_SESSION['newmemberusername'] = $_POST['newmemberusername'];
    $_SESSION['newmemberemail'] = $_POST['newmemberemail'];
    $_SESSION['newmemberpassword'] = $_POST['newmemberpassword'];


if(strlen($_POST['newmemberusername'])<3){
        header("Location:register.php?err=" . urlencode("The usernamename must be at least 3 characters long"));
        exit();
    }
    else if(strlen($_POST['newmemberpassword']) < 12){
         header("Location:register.php?err=" . urlencode("The password is less than 12 characters"));
        exit();
    }

    else if(!isUnique($_POST['newmemberemail'])){
        header("Location:register.php?err=" . urlencode("Email is already in use"));
        exit();
    }



    else {
        $newmemberusername = mysqli_real_escape_string($db , $_POST['username']);
        $newmemberemail = mysqli_real_escape_string($db , $_POST['email']);
        $newmemberpassword = mysqli_real_escape_string($db , $_POST['password']);


        $query = "insert into userregister (newmemberusername,newmemberemail,newmemberpassword) values('$newmemberusername','$newmemberemail','$newmemberpassword')";

        $db->query($query);
        $message = "
    Hi $newmemberusername! Account created on XXX Site



        mail($newmemberemail email , 'Activate Account' , $message , 'From:localhost');
        header("Location:index.php?success=" . urlencode("Activation Email Sent!"));
        exit();
    }

}
}
?>