A simple function to determine whether an IP is within a CIDR range or not.

cidr.php

<?php
/* A simple function to determine whether an IP is within a range
 * or not.
 *
 * Examples:
 *
 *   withinCIDR('1.2.3.4', '1.2.3.0/24');
 *   => 1
 *
 *   withinCIDR('1.2.3.4', '1.2.3.4');
 *   => 1
 *
 *   withinCIDR('1.2.3.4', '1.2.3.0/32');
 *   =>
 */
function withinCIDR($ip, $cidr)
{
    $within = false;

    if (strpos($cidr, '/')) {
        list($startIp, $prefix) = explode('/', $cidr);
        $count = 1 << (32 - $prefix);
        $start = ip2long($startIp);
        $end = $start + $count;

        $within = ip2long($ip) >= $start && ip2long($ip) < $end;
    } else {
        $within = $ip == $cidr;
    }

    return $within;
}