Nezaskodilo by ti precitat si aspon zaklady PHP. Napr. http://www.w3schools.com/php/
napr.
http://www.w3schools.com/php/func_filesystem_fopen .asp
http://www.w3schools.com/php/func_filesystem_fwrit e.asp
<?php
$file = fopen("test.txt","a");
$ip = $_SERVER['REMOTE_ADDR'];
fwrite($file,$ip."\n");
fclose($file);
?>
P.S. keby nefungoval riadok
fwrite($file,$ip."\n");
tak to rozpis na dva
fwrite($file,$ip);
fwrite($file,"\n");