curl POST format for CURLOPT_POSTFIELDS

Question

When I use curl via POST and set CURLOPT_POSTFIELD do I have to urlencode or any special format?

for example: If I want to post 2 fields, first and last:

first=John&last=Smith

what is the exact code/format that should be used with curl?

$ch=curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
$reply=curl_exec($ch);
curl_close($ch);