Ok, then try this variant of code:

$oPage = BxDolPage::getObjectInstanceByURI('', false, true);

if ($oPage) {

    $oTemplate = BxDolTemplate::getInstance();

    $oTemplate->setPageNameIndex (BX_PAGE_DEFAULT);

    $oTemplate->setPageType ($oPage->getType());

    $oTemplate->setPageInjections ($oPage->getInjections());

    $oTemplate->setPageHeader(/* new header */);

    $oTemplate->setPageContent ('page_main_code', $oPage->getCode());

    $oTemplate->getPageCode();

} else {

    $oTemplate = BxDolTemplate::getInstance();

    $oTemplate->displayPageNotFound();

}