Anti ddos php script

<?php


$ad_delay=2;

$ad_DirName=$_SERVER['DOCUMENT_ROOT'].'/tmp';


$ad_Robots_UserAgent=array(
   'aipbot',
   'Aport',
   'eStyleSearch',
   'Gigabot',
   'Gokubot',
   'Google',
   'MJ12bot',
   'msnbot',
   'PlantyNet_WebRobot',
   'StackRambler',
   'TurtleScanner',
   'Yahoo',
   'Yandex',
   'YaDirectBot',
);

$ad_good_ip = array(
    '217.107.36.73',
);


function ad_WiteIP($dir){
   $f=fopen($dir.'/a'.$_SERVER['REMOTE_ADDR'], 'w');
   fclose($f);
}

$ad_IsRobot=false;
foreach ($ad_Robots_UserAgent as $match){
   if (strstr($_SERVER['HTTP_USER_AGENT'], $match)){
      $ad_IsRobot=true;
      break;
   }
}
if( in_array($_SERVER['REMOTE_ADDR'], $ad_good_ip) ) {
     $good_ip = true;
} else {
     $good_ip = false;
}

if (!$ad_IsRobot AND !$good_ip){
   session_start();
}

if (!$ad_IsRobot AND !$good_ip){

   $ad_dir      =opendir($ad_DirName)
      or die('dir notfount );
   $ad_now      =time();
   $ad_forbid   =$ad_now-$ad_delay;

   WHILE (false!==($ad_FName=readdir($ad_dir))){
      if (ereg('^a[1-9]',$ad_FName)
         && (@ filemtime($ad_DirName.'/'.$ad_FName)<$ad_forbid)){
         @ unlink($ad_DirName.'/'.$ad_FName);
      }
   }
   closedir($ad_dir);

   if (file_exists($ad_DirName.'/a'.$_SERVER['REMOTE_ADDR'])){

      header('HTTP/1.0 503 Service Unavailable');
      header('Status: 503 Service Unavailable');
      header('Retry-After: '.$ad_delay*3);
?>
<!doctype html public "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Ошибка 503</title>
<meta http-equiv="Content-Type" content="text/html; charset=Windows-1251" />
</head>
<body>
<h1>Error 503 (Service Unavailable)</h1>
<p>Pless refreh page</p>
</body>
</html>
<?php
      ad_WiteIP($ad_DirName);
      exit;
   }else{
      ad_WiteIP($ad_DirName);
   }
}
?>