Using IP log to prevent DoS attack

<?php
    function UpdateLog() {
        chdir("iplog/");
        $ip=$_SERVER['REMOTE_ADDR'];
        $now = time();
        if (file_exists("$ip")) {
            $f = fopen($ip, "r");
            $last = fread($f, 255);
            fclose($f);
            
            $f = fopen($ip, "w");
            fwrite($f, $now);
            fclose($f);
            if ($now - $last < 2) {
                exit("I am tired!");
            } else {
                //echo "OK. You are welcome! " . date("h:i:s d/m/Y", $last);
            }
        } else {
            $f = fopen($ip, "w");
            fwrite($f, $now);
            fclose($f);
        }
    }
    UpdateLog();
    // main processing
?>