mainの記述形式
<!-- ghbinclude: relative filepath" -->
とでもするか.
$ghb_head = strlen("<!-- ghbinclude: ");
$ghb_tail = strlen(" -->");
$filepath = substr($line, $ghb_head, strlen($line)-$ghb_tail);
$fp = @fopen($filepath, "rt");
if($fp!=false){
/* 略 */
fclose($fp);
}