$PROXY_HOST = "10.10.20.30"; // Proxy server address
$PROXY_PORT = "8080";    // Proxy server port
$PROXY_USER = "myuser";    // Username
$PROXY_PASS = "mypass";   // Password

$auth = base64_encode("$PROXY_USER:$PROXY_PASS");
stream_context_set_default(
 array(
  'http' => array(
   'proxy' => "tcp://$PROXY_HOST:$PROXY_PORT",
   'request_fulluri' => true,
   'header' => "Proxy-Authorization: Basic $auth"
  )
 )
);

$proxy = "10.10.20.30:8080";
$useragent="Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1";
$url = "https://www.google.com";
$credentials = "myuser:mypass";

$ch = curl_init();
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT,15);
curl_setopt($ch, CURLOPT_HTTP_VERSION,'CURL_HTTP_VERSION_1_1' );
curl_setopt($ch, CURLOPT_HTTPPROXYTUNNEL, 1);
curl_setopt($ch, CURLOPT_PROXY, $proxy);
curl_setopt($ch, CURLOPT_PROXYUSERPWD,$credentials);
curl_setopt($ch, CURLOPT_USERAGENT,$useragent);
curl_setopt($ch, CURLOPT_URL, $url);
 curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_NTLM);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION,1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER,0);
$result=curl_exec ($ch);
curl_close ($ch);

echo 'RES: '.$result;