API and Databases in PHP

Often, an API is used to retrieve data from a database. Let's look at an example.

Suppose we have a table with users. Let's create an API that will take a user's id as a parameter and return the data from the database for that user:

api.loc/index.php<?php
  $id     = $_GET['id'];
  $query  = "SELECT * FROM users WHERE id=$id";
  $result = mysqli_query($link, $query);
  $user   = mysqli_fetch_assoc($result);
  
  header('Content-Type: application/json');
  echo json_encode($user, true);
?>

Задание 1

Suppose the database stores countries and their cities. Create an API that will take a country as a parameter and return an array of its cities.