PHP function for validating CIDR notation format (ipv4, ipv6)

validate_cidr.php

<?php 

/**
 * Validates the format of a CIDR notation string
 *
 * @param string $cidr
 * @return bool
 */
function validateCidr($cidr)
{
    $parts = explode('/', $cidr);
    if(count($parts) != 2) {
        return false;
    }

    $ip = $parts[0];
    $netmask = intval($parts[1]);

    if($netmask < 0) {
        return false;
    }

    if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {
        return $netmask <= 32;
    }

    if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
        return $netmask <= 128;
    }

    return false;
}

Thanks for the gist, saved me some work today. Improved netmask validation in my fork. Hope you can merge the changes.

The fork invalidates netmasks like: /, /22chars, /17.8.9.

Thank you!