file iplist.txt:
 192.168.0.0/24
 172.16.0.0/16
 10.0.0.0/8

<?php
########### ncritten's function myip2long

function myip2long($ip) {
    if (is_numeric($ip)) {
        return sprintf("%u", floatval($ip));
    } else {
        return sprintf("%u", floatval(ip2long($ip)));
    }
 }

########### function to chek ip if it in one of denyied/allowed networks =)

function ipfilter($ip) {
    $match = 0;

### converting ip address in binary
    $ip_addr = decbin(myip2long($ip));

### the file wich contains allowed/denyied networks
    if (fopen("iplist.txt", "r")) {
        $source = file("iplist.txt");

        foreach ($source as $line) {

### exploding each network to obtaid network address and cidr
            $network = explode("/", $line);
            $net_addr = decbin(myip2long($network[0]));
            $cidr = $network[1];

### and finaly cheking quantity of network bits from left to right wich is equal to cidr is equal to the same bits of ip address
            if (substr($net_addr, 0, $cidr) == substr($ip_addr, 0, $cidr)) {
                $match = 1;
                break;
            }
        }
    }
    return $match;
 }

### this function will return 1 if IP match to some network or 0 if will not match

 ### and finaly the chek will be like this

$user_ip = $_SERVER['REMOTE_ADDR'];

 if (ipfilter($user_ip) == 1)  echo "allowed!";
 else echo "deny!";

?>