Authorization: Basic [client_id]:[client_secret]
Content-Type: application/x-www-form-urlencoded

Authorization: Basic bXlfY2xpZW50X2lkOnBFUnkyTGhLYko0U2FkY3ZLcklpQW5xWnprakg5bm9STUc3aUxZcWl2MA==

grant_type=password&scope=read write&username=[username]&password=[password]

{
    "access_token":"eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9(...)",
    "token_type":"Bearer",
    "expires_in":3600,
    "refresh_token":null
}

$api = "KEY GOES HERE";
$authurl = "https://url.com/oauth/token";

$client_id = "ID GOES HERE";
$client_secret = "SECRET GOES HERE";

// Creating base 64 encoded authkey
$Auth_Key = $client_id.":".$client_secret;
$encoded_Auth_Key=base64_encode($Auth_Key);

$headers = array();
$headers['Authorization'] = "Basic ".$encoded_Auth_Key;
$headers['Content-Type'] = "application/x-www-form-urlencoded";

$data = "grant_type=password&scope=read write&username=".$api."&password=".$api."";

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $authurl);
curl_setopt($ch, CURLOPT_POST, 1 );
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_TIMEOUT, $timeout);

$auth = curl_exec( $ch );

if ( curl_errno( $ch ) ){
    echo 'Error: ' . curl_error( $ch );
}
curl_close($ch);

$secret = json_decode($auth);
$access_key = $secret->access_token;