Ack! I was typing in a hurry and missed the last bit that needed to be removed:
: ''
at the end of the statement. This will work:
PHP Code:
<?php echo '<a id="cartButton" href="' . zen_href_link(FILENAME_SHOPPING_CART, '', 'SSL') . '" title="' . CART_HEADER_TITLE . '"><img src="whatever.png" /></a>'); ?>