Call to object function, want to PHP interpret returned string

Hi At the top of a webpage I have: <?php include_once("furniture.php"); $myFurniture = new furniture(); echo $myFurniture->getTop("my company title"); ?> to deliver the first lines of HTML, everything in HEAD and the first bits of page furniture (menu, etc). In the furniture object in getTop(), I want to return a string that includes the CSS file that I call with an include_once. But the include_once isn't interpreted by PHP, it's just outputted. So from:

       $toReturn = "<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Transitional//EN' ........
           <?php
               include_once('styles3.txt');
           ?>
           .......";

       return $toReturn;

I get <?php include_once('styles3.txt'); ?> in my code. Do I really have to break up my echo $myFurniture->getTop("my company title"); call to getTopTop, then include my CSS, then call getTopBottom, or can I get PHP to interpret that text that came back? PS. I may be stupid, this may be obvious .. I don't program PHP every day Thanks in advance for your help :-) Cheers J