FPDF is a PHP class which allows to generate PDF files with pure PHP. F from FPDF stands for Free: you may use it for any kind of usage and modify it to suit your needs.
I did try to replicate exactly as you do, but i get fatal error collision on construct method, a change the name of the method, i don't know if that is the right solution
I did try to replicate exactly as you do, but i get fatal error collision on construct method, a change the name of the method, i don't know if that is the right solution