uploadToken里的四个参数分别是什么意思？

uploadToken($bucket);

$policy = array(
    'returnUrl' => 'http://127.0.0.1/demo/simpleuploader/fileinfo.php',
    'returnBody' => '{"fname": $(fname)}',
);
$upToken = $auth->uploadToken($bucket, null, 3600, $policy);

echo $upToken;

uploadToken($bucket);

$policy = array(
    'returnUrl' => 'http://127.0.0.1/demo/simpleuploader/fileinfo.php',
    'returnBody' => '{"fname": $(fname)}',
);
$upToken = $auth->uploadToken($bucket, null, 3600, $policy);

echo $upToken;