PHP: How to render PDF reports containing diagrams?

I want to create PDF reports containing some pie / bar charts. I have to create them using my PHP web application and cannot use command line tools. For the interface I use flot . However, I do not know how to get these diagrams in PDF, because my project is limited to PHP.

How to create images from my charts for PDF rendering?

Update. The solution should be open source, as my project will also be open.

Best

Stephen