MapMyFitness API - can't get OAuth2 working in PHP

All,

I'm attempting to use MapMyFitness' API and OAuth2.

Here's the stripped down code I'm using (similar to code I've used successfully to connect with Strava's and RunKeeper's API):

$mapMyRun_authorization_code = "*****";
$client_id = "*********";
$client_secret = "*************";

$url="https://oauth2-api.mapmyapi.com/v7.0/oauth2/access_token/";

$postfields = array(
    "grant_type" => "authorization_code",
    "client_id" => $client_id,
    "client_secret" => $client_secret,
    "code" => $mapMyRun_authorization_code
);

$headers = array('Api-Key: ' . $client_id);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); 
curl_setopt($ch, CURLOPT_POSTFIELDS, $postfields);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$json = curl_exec ($ch);
$responsecode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

When I run this - $json is empty, and $responsecode is 400, not 200.

I'm obviously doing something wrong with my request, but I have no idea what...

Solution

So, thanks to help from the MapMyRun team, here's the answer:

$mapMyRun_authorization_code = "*****";
$client_id = "*********";
$client_secret = "*************";

$url="https://oauth2-api.mapmyapi.com/v7.0/oauth2/access_token/";

$postfields = "grant_type=authorization_code&code=" . $mapMyRun_authorization_code . "&client_id=". $client_id . "&client_secret=" . $client_secret;

$headers = array('Api-Key: ' . $client_id);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); 
curl_setopt($ch, CURLOPT_POSTFIELDS, $postfields);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$json = curl_exec ($ch);
$responsecode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

The important changes - $postfields cannot be an array, and you should not include curl_setopt($ch, CURLOPT_POST, true);

Hopefully this will be helpful to someone else...