php - jquery ajax post doesn't work

The jQuery code is sending the POST data as a JSON string, but your PHP code is looking for standard key-value pairs.

You can change the Ajax call to send in key-value format. I can't see what utnew refers to in your code, usually that would be a Javascript object or the result of serializing an HTML form with $.serialize():

$.post("myurl.php", utnew, "json")
  .always(function(response) { console.log(response); }

Or you can change the PHP side to expect a JSON-encoded string in the POST payload rather than encoded key-value pairs:

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

    if (isset($data['id'])) { ... }

Whichever you choose, the problem you're having is the Ajax call sending a JSON-encoded string as the POST payload, but PHP expecting key-value pairs in $_POST.

The "json" data type argument to jQuery $.ajax or the shortcut method $.post tells jQuery what format to expect the return value in, so your PHP code should return a JSON-encoded string. Your PHP code is sending back a plain string, so you should either tell jQuery to expect that:

$.post(url, data, "text")

or have PHP send back a proper JSON-encode object:

echo json_encode(["result" => "success"]);

If you have PHP error logging on (you should) to a file you can debug your API endpoint with error_log, i.e. error_log(print_r($_POST,1)); would have given you some clues.