https://www.php.net/manual/en/wrappers.php.php
php://input is a read-only stream that allows you to read raw data from the request body. In the case of POST requests, it is preferable to use php://input instead of $HTTP_RAW_POST_DATA as it does not depend on special php.ini directives. Moreover, for those cases where $HTTP_RAW_POST_DATA is not populated by default, it is a potentially less memory intensive alternative to activating always_populate_raw_post_data. php://input is not available with enctype=“multipart/form-data” .
The line file_get_contents("php://input"); loads data in that was passed to the program thats not presented as $_GET or $_POST data but in the BODY of the request into PHP (for example)