Another option to generate PDF from PHP without installing external libraries and non-free software is "PDFnow".
It works based on a powerful template mechanism, especially useful for letters, invoices, and manuals.
The PDF functions in PHP can create PDF files using the PDFlib library from PDFlib GmbH (» www.pdflib.com). A restricted version called PDFlib Lite 7 is available for free, but it is no longer maintained since 2010. PDFlib integration for PHP is still actively maintained by PDFlib GmbH. It is mainly targeted at the commercial PDFlib versions, but can also be used for the old PDFlib Lite.
If you want to use PDFlib we urge you start with the examples which are included in all PDFlib packages, and refer to the extensive manuals in PDF format which are also included. At a minimum you should take a look at the hello.php sample to learn basic setup for PDFlib usage.
If you're interested in alternative free PDF generators that do not utilize external PDF libraries, see this related FAQ.