Evaluating PHP

I have developed a module and part of it includes parsing templates in the backend. I am having trouble evaluating the PHP within templates. I have the following code which I thought would do the trick but I’m not getting anywhere:

global $FNS;
ob_start();
echo $FNS->evaluate($str);    
$str = ob_get_contents();
ob_end_clean();

Can anyone point me in the right direction? I’m using EE 1.6.7 and PHP 5.