This will log each ip into a text file, does not check for duplicates.
PHP:
<?php
$ip = $_SERVER['REMOTE_ADDR'];
$d = date("d-m-y / H:i:s");
$log = $d . " -- " . $ip ."<br />";
$fopen = fopen("logs.html", "a");
fwrite($fopen, $log);
fclose($fopen);
?>