Where did I make a mistake in json?

Question

AJAX

$("#form-phone").submit(function(e) {
                e.preventDefault();

                var phone= $("#phone").val();

                $.ajax({
                    type: "POST",
                    url: "json.php",
                    data: $(this).serialize(),
                    cache: false,
                    success: function(response) {
                        var jsonData = JSON.parse(response);

                        if (jsonData.ok == "1") {
                            alert("ok");
                        }else{
                            alert("error");
                        }
                    }
                });
            });

if(isset($_POST['phone'])) {
  $phone= $_POST['phone'];
        echo json_encode(array('ok' => 1));
}else{
  echo json_encode(array('ok' => 0));
}

VM1632:1 Uncaught SyntaxError: Unexpected token < in JSON at position 0

Answer 1

Open the developer tools, network, and see what your Ajax request returns, most likely you have some kind of warning there, or an error.
In addition, parsing the string is not necessary at all, it is enough to specify the type of the returned value in Ajax.

Answer 2

There may be some character at the beginning of the line that is not related to json. Try first to remove the first character from the string, then JSON.parse later. Before that, it's better to check what returns. Because if an error, then the first character has nothing to do with