No value when call get_one

[nd3w]

When I call http://localhost/php_rest_api_without_framework/api/get.php I receive all students table content.
But when I call http://localhost/php_rest_api_without_framework/api/get_one.php unlike the screenshot in (readme.md) I got Error: Student ID is missing! message.

It's like no result from get_one.php at line 15:

    $data = json_decode(file_get_contents("php://input"));