The quickest and easiest way to generate a PDF is to make use of an existing PDF library. Search the web for "PDF library for PHP" and you'll discover there a number of libraries for PHP out there, both paid and free. Pick one that suits your needs.
Policies can sometimes restrict what you're allowed to install on a server. If that's the case, using an online PDF service might be an option, assuming you're not working with sensitive data. Their APIs are normally language agnostic - whether you're using PHP or some other language shouldn't be an issue.
Creating PDFs from scratch using just PHP is the hard way and generally best avoided, but if you're interested in the format the PDF reference can be found here on Adobe's site.