DOCXPath supports setting document, header and footer as targets. We have done some tests and we get the information from headers and footers correctly. Since phpdocx 10 the Indexer class also extracts alt text and description contents from images.
We recommend you to download the latest version available on MY PHPDOCX page after login, and check the included sample Docxpath/getDocxPathQueryInfo/sample_4.php that gets information from headers:
$referenceNode = array(
'target' => 'header',
'type' => 'paragraph',
$queryInfo = $docx->getDocxPathQueryInfo($referenceNode);
If you open a support ticket (https://www.phpdocx.com/support) attaching the DOCX you are using, the dev team will generate a custom script using your document.