know the answere of user to the telegram bot

i am a new programmer in the telegram bot i have a php code for my bot

const TOKEN = "https://api.telegram.org/bot<TOKEN>";
const USER_NAME = "@testbot";
const NAME = "Test bot";
$jata = json_decode(file_get_contents("php://input"), true);

define("CHAT_ID", $jata["message"]["chat"]["id"]);
define("MESSAGE", $jata["message"]["text"]);

switch (MESSAGE) {
    case "/start":
        SendMessage("welcome to bot");
    break;
    case "/new":
        SendMessage("write your name: ");
    break;
    default:
        // ????????????????????????????????????????
}

function SendMessage($msg)
{
    $ch = curl_init();
    curl_setopt_array($ch, array(
        CURLOPT_URL => TOKEN . "/sendMessage",
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => true,
        CURLOPT_POSTFIELDS => array(
            "chat_id" => CHAT_ID,
            "text" => $msg
        )
    ));
    curl_exec($ch);
    curl_close($ch);
}

so i want when the user write /new my bot send to user write your first-name and then user sent to me i send to user enter your last-name bot how can i know the answered to which one? i dont know what i should write in default in switch block to know the user sent me first-name or last-name

You can create a file called {user_id}.json with the user_id of the user who sent the /new command and write in it a json object with this structure:

{
  "first_name": "",
  "last_name": "",
}

and when the user send to the bot his first-name, the bot check if the first_name key of the json object is an empty string, if so replace it with the text of the message sent by the user and ask for the last-name. When the user send the last-name, you can check if the first-name is empty (it will not be) and so you know that the user has already sent the first-name, so you can store the last-name in the respective key.

Code:

<?php
const TOKEN = "https://api.telegram.org/bot<TOKEN>";
const USER_NAME = "@testbot";
const NAME = "Test bot";
$jata = json_decode(file_get_contents("php://input"), true);

define("CHAT_ID", $jata["message"]["chat"]["id"]);
define("MESSAGE", $jata["message"]["text"]);

$user_id = $jata["message"]["user"]["id"];


switch (MESSAGE) {
    case "/start":
        SendMessage("welcome to bot");
    break;
    case "/new":
        SendMessage("write your name: ");
        // Create a json object
        $json = [
            "first_name" => "",
            "last_name" => "",
        ];
        // Store the json object in a file called {user_id}.json
        file_put_contents("$user_id.json", json_encode($json));
    break;
    default:
        // Check if the file with the user's data exists and the message text is not empty (the message text is empty with media messages like photos, videos...)
        if (file_exists("$user_id.json") && !empty(MESSAGE)){
            // Read the user's data and decode it in an array
            $data = json_decode(file_get_contents("$user_id.json"), true);
            // Check if the first_name key in the array is an empty string
            if ($data['first_name'] == ""){
                // If yes replace the first_name key of the object with the message content
                $data['first_name'] = MESSAGE;
                SendMessage("write your surname: ");
            }else{
                // If not the user has already sent the first-name and he just sent the last_name
                $data['last_name'] = MESSAGE;
                $first_name = $data['first_name'];
                $last_name = $data['last_name'];
                SendMessage("Your name is $first_name $last_name");
            }
            // In any case, store the array with the user's data to the file
            file_put_contents("$user_id.json", json_encode($data));
        }
}


// To get the first_name from a user by his user_id
if (file_exists("$user_id.json")){
    $user_info = json_decode(file_get_contents("$user_id.json"), true);

    $first_name = $user_info['first_name'];
    $last_name = $user_info['last_name'];
}

function SendMessage($msg)
{
    $ch = curl_init();
    curl_setopt_array($ch, array(
        CURLOPT_URL => TOKEN . "/sendMessage",
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => true,
        CURLOPT_POSTFIELDS => array(
            "chat_id" => CHAT_ID,
            "text" => $msg
        )
    ));
    curl_exec($ch);
    curl_close($ch);
}