PHP curl request with url in variable

I'm going crazy with this. I read some other question about it, but never found an answer.

My problem is: I'm trying to create a PHP Curl request (GET), and the request return false if I set the url from a variable.

Here below my code:

Working code:

$ch = curl_init ();
curl_setopt ($ch, CURLOPT_URL, "www.google.com");
curl_setopt ($ch, CURLOPT_COOKIEFILE, $ckfile); 
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt ($ch, CURLOPT_PORT, 80);
$output = curl_exec ($ch);
echo json_encode($output);

Not working code:

$url = "www.google.com";
$ch = curl_init ();
curl_setopt ($ch, CURLOPT_URL, $url);
curl_setopt ($ch, CURLOPT_COOKIEFILE, $ckfile); 
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt ($ch, CURLOPT_PORT, 80);
$output = curl_exec ($ch);
echo json_encode($output);

Someone could help me with this? Thank you.