Recently I was asked to generate PDF invoices for an online shop. I looked at various PHP PDF generators, but wasn't particularly impressed with any of them.
Then I found (via Stack Overflow) a command-line HTML-to-PDF convertor called wkhtmltopdf, which uses WebKit (the same layout engine as Safari and Google Chrome) and therefore is very accurate.
There is a class for PHP integration on the Wiki, but I found it overly complicated and it uses temp files which aren't necessary. This is the code I wrote instead.
I used Smarty for generating the HTML for the PDF, but you can use any template engine, or pure PHP if you prefer.
Note: I originally tried to install wkhtmltopdf from source, but it's much easier to use the static binary instead.