PayPal IPN in PHP cannot send request to IPNPB

Question

I have here a PayPal IPN script, but the problem is, it is receiving the POST request correctly, but when it tries to make a CURL request to PayPal, it seems that no response is given by PayPal (not even INVALID or anything!)

Here is my code:

<?php
    // Format POST values
    foreach($_POST as $name => $value) {
        $data .= $name.'='.$value.'&';
    }
    $data = substr_replace($data, NULL, -1);
    $data = urlencode($data);


    // THE PROBLEM COMES HERE, I TRY TO MAKE A CURL REQUEST TO PAYPAL BUT NO RESPONSE RETURNED
    $ch = curl_init('https://sandbox.ipnpb.paypal.com/cgi-bin/webscr');
    curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
    curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Connection: Close'));

    $res = curl_exec($ch);
    $res = curl_close($ch);

    file_put_contents('result.txt',$res);
    // There is nothing in my result.txt file for $res , but $data returns the POST data PayPal gave me
?>