Hello,
You can get the node element using the getDOCXPathQueryInfo method (https://www.phpdocx.com/api-documentation/docx-path/get-docx-path-query-info) querying by the placeholder. This method returns an array with three keys: elements, length and query. You can get the XML value of the DOM node through the elements content, do the needed changes to the XML and replace the source node by the one as raw WordML content (https://www.phpdocx.com/api-documentation/word-content/add-raw-wordml-Word-document).
Regards.