curl_setopt($ch, CURLOPT_URL, 'http://127.0.0.1:4444/wd/hub/session/'.$session_id.'/url' );
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode(array( 'url'=>'http://www.ya.ru' )));
http://127.0.0.1:4444/wd/hub/session/$session_id/screenshot
и в ответ придет JSON с base64 скришотом…