Checking Login Availability

Currently, our registration has one problem - a new user of our site can register with an already existing login, which, of course, is unacceptable.

To solve the problem, it is necessary before the request to add a new user to the database, execute a SELECT query that will check whether the desired login is taken or not. If not taken - register, if taken - do not register, but display a message about it.

Let's write this code:

html<?php
  if (!empty($_POST['login']) and !empty($_POST['password'])) {
    $login = $_POST['login'];
    $password = $_POST['password'];
    
    $query = "SELECT * FROM users WHERE login='$login'";
    $user = mysqli_fetch_assoc(mysqli_query($link, $query));
    
    if (empty($user)) {
      $query = "INSERT INTO users SET login='$login', password='$password'";
      mysqli_query($link, $query);
      
      $_SESSION['auth'] = true;
      
    } else {
      // login is taken, display a message about it
    }
  }
?>