mysql get this inserted id php

1<?php
2$servername = "localhost";
3$username = "username";
4$password = "password";
5$dbname = "myDB";
6
7// Create connection
8$conn = mysqli_connect($servername, $username, $password, $dbname);
9// Check connection
10if (!$conn) {
11    die("Connection failed: " . mysqli_connect_error());
12}
13
14$sql = "INSERT INTO MyGuests (firstname, lastname, email)
15VALUES ('John', 'Doe', 'john@example.com')";
16
17if (mysqli_query($conn, $sql)) {
18    $last_id = mysqli_insert_id($conn);
19    echo "New record created successfully. Last inserted ID is: " . $last_id;
20} else {
21    echo "Error: " . $sql . "<br>" . mysqli_error($conn);
22}
23
24mysqli_close($conn);
25?>

1$this_id = mysqli_insert_id($db);