$data = array(
    'client_id' => 'testtest',
    'client_secret' => 'TheBigSecret',
    'redirect_uri' => 'https://oa.dnc.global/web/?action=oauth',
    'grant_types' => 'authorization_code',
    'scope' => 'openid sli' 
);

$h = curl_init("https://oa.dnc.global/web/http.api/json/clients/");
curl_setopt($h, CURLOPT_RETURNTRANSFER, true);
curl_setopt($h, CURLOPT_TIMEOUT, 10);
curl_setopt($h, CURLOPT_POST, true);
curl_setopt($h, CURLOPT_HTTPHEADER, array('
    Content-Type: application/x-www-form-urlencoded'),
    'Accept: application/json'
);   
curl_setopt($h, CURLOPT_POSTFIELDS, http_build_query($data));

$res = curl_exec($h);
...