Writing/Drawing over a PDF template document in PHP

Have a look at the FPDI Library an add on to FPDF for template annotation.

It can also bolt-on to TCPDF, another popular PHP PDF library. An existing PDF is used as the base of a page, instead of a blank, after that the procedures are the same as regular PDF creation.

When you want to use PDF lib to write over a document, positionning is time consuming and boring. Dhek is useful for such task: https://github.com/cchantep/dhek/releases .

JSON mapping over PDF page/coordinates is defined, so that you can write with PDF API you prefer.

You probably want to use PDF Forms for what you want to do. To fill these babies you could use the FDF method described here: Using HTML forms to fill in PDF fields with PHP and FDF.
There is actually another nice SO post about PDF form filling here: Filling PDF Forms with PHP.