Код:
<?
ob_start();
include (!empty($content) && preg_match('/^[\w\/]{1,255}$/',$content) && file_exists($content.'.inc'))?$content.'.inc':'default.inc';
echo preg_replace('/[\r\n]+/','',ob_get_contents());
ob_end_clean();
?>
page.php?content=something -> something.inc
page.php?content=something/sub/sub -> something/sub/sub.inc
...