发现$_POST 没数据，而$_REQUEST 有数据
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https://stackoverflow.com/questions/26928393/php-post-is-empty-but-http-raw-post-data-has-all-data
The POST data must be in query string or multipart/form-data format to get decoded properly. Your data seems to be JSON, so you have to decode it by yourself:
$_POST = json_decode(file_get_contents('php://input'), true);