should be:
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
'Accept: application/vnd.twitchtv.v3+json',
'Authorization: OAuth ' .$_SESSION['user']['token']
));
The call to CURLOPT_HEADER controls if the HTTP HEADERS are passed back from the Request to curl_exec (as further more directed by CURLOPT_RETURNTRANSFER)
Please see the documentation