send_form.php

<?php
require_once('recaptchalib.php');

// Get a key from https://www.google.com/recaptcha/admin/create
$publickey = "6Ld6m-8SAAAAAGNrxIf7Dn6rt_6yfjcMO40Zxs_G";
$privatekey = "6Ld6m-8SAAAAAOLkKK6T8j4K9YntpGA0f-xszASW";
$token = 'VopcWwRNfnCzUhmgaXM7sbQBurqixY59yFvt1J8ZkjeD2AK4SH36TPLdEG';
$referer = "http://www.verscontabilidade.com.br";
$url_back = "http://www.verscontabilidade.com.br/obrigado";

# the response from reCAPTCHA
$resp = null;
# the error code from reCAPTCHA, if any
$error = null;

function sendMessage($post, $publickey, $privatekey, $token, $referer, $url_back){
  if($post['recaptcha_response_field']){
    if(verifyCaptcha($privatekey, $post)){
        $nome = $_POST['nome'];
        $email = $_POST['email'];
        $assunto = $_POST['assunto'];
        $mensagem = $_POST['mensagem'];

        $url = "http://enviarformularios.com.br/send";

        $fields = array(
            'nome' => $nome,
            'email' => $email,
            'assunto' => $assunto,
            'mensagem' => $mensagem,
            'token' => $token
        );

        //url-ify the data for the POST
        foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
        rtrim($fields_string,'&');

        //open connection
        $ch = curl_init();

        //set the url, number of POST vars, POST data
        curl_setopt($ch,CURLOPT_URL,$url);
        curl_setopt($ch,CURLOPT_POST,count($fields));
        curl_setopt($ch,CURLOPT_POSTFIELDS,$fields_string);
        curl_setopt($ch,CURLOPT_AUTOREFERER,true);
        curl_setopt($ch,CURLOPT_REFERER, $referer);

        //execute post
        $result = curl_exec($ch);
        if($url_back){
          header("location: ".$url_back);  
        } else {
          echo "<script>alert('Formulario enviado com sucesso!')</script>";    
        }
    } else {
      echo "<script>alert('Captcha incorreto.')</script>";
    }
  } else {
    echo "<script>alert('Favor preencher o captcha')</script>";
  }
}

function verifyCaptcha( $privatekey, $post ) {
  $resp = recaptcha_check_answer ($privatekey,
    $_SERVER["REMOTE_ADDR"],
    $post["recaptcha_challenge_field"],
    $post["recaptcha_response_field"]);
  return $resp->is_valid;
}

if ($_POST){
  sendMessage($_POST,$publickey,$privatekey,$token,$referer,$url_back);
}
?>


...

<form method="POST">

...
<?php 
  # Captcha
  echo recaptcha_get_html($publickey, $error); 
?>

</form>