<pre>
//请求有效时间 默认 3 分钟
$expiration = str_replace('+00:00', '.000Z', gmdate('c', time() + 180));
//限制上传大小 默认 10MB
$conditions = [
[0 => 'content-length-range', 1 => 0, 2 => (int)$config['maxSize'] * 1024 * 1024],
[0 => 'starts-with', 1 => '$key', 2 => $dir],
];
$policy = json_encode(['expiration' => $expiration, 'conditions' => $conditions]);
</pre>