Retrieving Facebook Marketing API leads
since you are using curl, try doing something like this and see if this returns any results
$ret = exec("curl -G -d 'limit=100' -d 'access_token=" . $access_token . "' \ -d 'filtering=[{\"field\":\"time_created\",\"operator\":\"GREATER_THAN\",\"value\":\"" . date("Y-m-d", time() - (24 * 3600 * 30)) . "T" . date("H:i:s", time() - (24 * 3600 * 30)) . "+0000\"}]' \ https://graph.facebook.com/v2.8/" . $form_id . "/leads");