gistfile1.php

<?php
    // fill in with network addresses in CIDR notation:
    $bans = array(
        'xxx.xxx.xxx.xxx/yy'
    );
    
    function checkban($ipstring) {
        global $bans;
        $ip = ip2long($ipstring);
        foreach ($bans as $ban) {
            $parts = split("/", $ban, 2);
            $net = ip2long($parts[0]);
            $cidr = (int)$parts[1];
            if (($net >> $cidr) == ($ip >> $cidr))
                return true;
        }
        return false;
    }
    
    if (checkban($_SERVER['REMOTE_ADDR'])) {
        // BANNED USER
    }
    else {
        // NORMAL USER
    }
?>