php curl_exec returns empty

curl_exec will return false when the request failed. Adjust your function to this :

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_PROXY, $proxy); // $proxy is ip of proxy server
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
curl_setopt($ch, CURLOPT_TIMEOUT, 10);

$httpCode = curl_getinfo($ch , CURLINFO_HTTP_CODE); // this results 0 every time
$response = curl_exec($ch);

if ($response === false) 
    $response = curl_error($ch);

echo stripslashes($response);

curl_close($ch);

This way u'll see the curl error

You're trying to access a HTTP response code before you actually make the HTTP call. Reverse the execution as follows:

$response = curl_exec($ch);
$httpCode = curl_getinfo($ch , CURLINFO_HTTP_CODE); // this results 0 every time