how to input a png watermark image on a pdf file then download it….in PHP

I’m using PHP can someone help me, the flow was the user need to choose a file first then after choosing the file the program need to add a PNG watermark signature then after that the user can print or download the file…someone tells me that I need to convert the pdf file to PNG file after converting it, it will insert a PNG signature file then convert it back to pdf…im newbie when it comes to programming thanks…