BlackhawkG7

// Get the root page in current page tree
$pid = intVal($_GET['id']);
$page = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Frontend\\Page\\PageRepository');
$root = array_pop($page->getRootLine($pid));

BlackhawkG7

// get 10 levels of the tree recursively starting at $pid
$pid = intVal($_GET['id']);
$queryGenerator = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Database\\QueryGenerator');
$pidList = $queryGenerator->getTreeList($pid, 10, 0, 1);
$pids = explode(',',$pidList);