Originally Posted by holograph

Your code:
define('TEXT_MAIN', '' . tep_image(DIR_WS_IMAGES . 'default/greeting.jpg') . '<hr width="375" size="1" color="#000000"><br><?php include("main_content.php"); ?>');

Try this instead:
define('TEXT_MAIN', '' . tep_image(DIR_WS_IMAGES . 'default/greeting.jpg') . '<hr width="375" size="1" color="#000000"><br>'.include("main_content.php")) ;