This is not very practical, but for completeness: If you already have an ODT template, then you might very well retain that as template. Modifying the OpenDocument content.xml and replacing placeholders therein is pretty simple. If so, you could use unoconv or pyodconverter to transform the ODT into a final PDF.
unoconv -f pdf -o final.pdf template.odt
Very obviously this requires a full OpenOffice setup (UNO and Writer) on the webserver. And obviously not every webhoster would go with that! haha. Even if it's simple on any Debian or Fedora setup. The execution speed would probably not be stellar either. But then it might be the cleanest approach, since OOo governs both formats way better than any PHP class ever could.