PHP curl timeout error detection

I use curl to perform a HTTP request like this:

$ch = curl_init();
$timeout = 5;
curl_setopt($ch,CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch,CURLOPT_CONNECTTIMEOUT,$timeout);

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

How can I check if an error occurred, and whether it was a timeout error?