<?php
ini_set("display_errors", 0); // make sure there is no unexpected output while in production mode
$theIP = $_SERVER["REMOTE_ADDR"];
$ips = "ips.txt"; // a file writable by the web server
$list = file($ips);

foreach ($list as $key => $ip) {
    $list[$key] = trim($ip);
}

if (!in_array($theIP, $list)) {
    array_push($list, $theIP);
    mail(
        "you@somesite.com",
        "New IP Address",
        "{$theIP} -> " . gethostbyaddr($theIP),
        "From: me@mycomputer.com"
    );
    exec("echo '{$theIP}' >> {$ips}");
}
?>