How to ban / block IP addresses

This example gets the users IP address and exits the page if it matches a banned number.
 

<?php

//Set banned IP
$banned  = '123.123.123.123';

//get user IP address
$userip = $_SERVER['REMOTE_ADDR'];

//check for banned IP address
if($userip == $banned){
    echo "<BR>This IP address has been banned";
    exit();
}

//continue with script...