There are 2 ways to do this: you can take the text of the first text element of the root: $twig->root->first_child( "#TEXT")->text, or you can simply use the text_only method: $twig->root->text_only.

In both case you might need to trim the result from the extra spaces you get, which in the first option can be done simply by writing $twig->root->first_child( "#TEXT")->trimmed_text