I commented this line out with a // but after that ?> is interpreted as an end of script remark. Ofcourse this shouldn't happen, to my opinion in a comment nothing should be parsed.

<?
// onLoad="Javascript:parent.main_view.location='<? echo $base_url . $search_page; ?>';"
?>

the output will be:

';" ?>

But if I run:

<?
/* onLoad="Javascript:parent.main_view.location='<? echo $base_url . $search_page; ?>';" */
?>

there will be no output.

I ran this through PHP in CGI mode. It does not happen to comments within /* */ comment markers.