PHP Opening tag -- <? and <?php

[eluser]foobarph[/eluser]
Hi CI guys, I have a quick newbie question (I tried searching about it the net and the forum but i cannot seem to find answer)

How does '<?=$error ?>' work with CI and PHP?

I mean, my PHP doesn't compile this stuff. Is there any substitute for me to make this work?

Concrete examples are below.

<?=form_open_multipart('upload/do_upload'); ?>
- does not work on my part

<?php echo form_open_multipart('upload/do_upload'); ?>
- works on my part.

If there is a setting in my PHP file that I should change, please tell me. Thanks!