drupal_is_front div changer
Actually there is no function (built into core, at least) called drupal_is_front(). I believe what you're looking for is drupal_is_front_page() and in your case you could use it like this:

&lt;div id="content_&lt;?php if (drupal_is_front_page()): print 'main'; else: print 'something_else'; endif; ?&gt;"&gt;
  ....
&lt;/div&gt;


Or

&lt;?php
if (drupal_is_front_page()) {
  $divid = 'content_main';
}
else {
  $divid = 'content_something_else';
}
?&gt;
  &lt;div id="&lt;?php print $divid; ?&gt;"&gt;
    ....
  &lt;/div&gt;


I think it's also worth mentioning that if you're applying the code to a template file, you should also be able to use the variable $is_front in place of the function drupal_is_front_page(). The difference might be minuscule, but using a variable rather than a function should save you a little bit of overhead on your server.