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:

<?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
		}
	}
?>

Modify your code so that when attempting registration, a check for login availability is performed and, if it is taken, - display a message about it and ask to enter another login.