Retrieving Slack messages using PHP and cURL

I am following the instructions at https://api.slack.com/messaging/retrieving#getting_started to retrieve Slack messages using PHP and cURL. I can list my channels ok but trying to retrieve messages generates an error {"ok":false,"error":"not_in_channel"}. This error is not listed on the method page at https://api.slack.com/methods/conversations.history#response so I am puzzled as to what is wrong.

//Initialize cURL.
$ch = curl_init();

//Set the URL that you want to GET by using the CURLOPT_URL option.
curl_setopt($ch, CURLOPT_URL, 'https://slack.com/api/conversations.history');

curl_setopt($ch, CURLOPT_HTTPHEADER, array(
  'Authorization: Bearer xoxb-mykeyishereinmycode' 
));

//Set CURLOPT_RETURNTRANSFER so that the content is returned as a variable.
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

//Set CURLOPT_FOLLOWLOCATION to true to follow redirects.
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);

//Set CURL Data
$fields = array(
    'channel' => 'C041N9P069J'
);
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);

//Execute the request.
$data = curl_exec($ch);

//Close the cURL handle.
curl_close($ch);

print_r($data);