[PHP] Check if Client IP within CIDR

 if (!empty($_SERVER['HTTP_CLIENT_IP'])) {
    $ip = $_SERVER['HTTP_CLIENT_IP'];
} elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
    $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
} else {
    $ip = $_SERVER['REMOTE_ADDR'];
};

if (cidr_match($ip,"192.168.0.0/16") == True) {
echo "hit";
} else {
echo "pass";
};

function cidr_match($ip, $range) {
    list ($subnet, $bits) = explode('/', $range);
    if ($bits === null) {
        $bits = 32;
    }
    $ip = ip2long($ip);
    $subnet = ip2long($subnet);
    $mask = -1 << (32 - $bits);
    $subnet &= $mask; # nb: in case the supplied subnet wasn't correctly aligned
    return ($ip & $mask) == $subnet;
};

Origin: https://stackoverflow.com/questions/594112/check-whether-or-not-a-cidr-subnet-contains-an-ip-address