Hello,
What version and license of phpdocx are you using? placeholders are added as text strings (or alt text for images), but a link is not added as a text string but an internal XML content. You can add a text placeholder (not a link) and then replace it by a WordFragment link with all licenses; or use the rawSearchAndReplace method to replace internal placeholders, this method is available in Advanced and Premium licenses.
On the rawSearchAndReplace API page you can see an example about replacing a link with an internal placeholder.
Regards.