Generally speaking, it is fairly rare to find more than one commonly used way to solve a PHP problem involving a specific piece of external functionality. Originally this was true of creating Flash movies - one could use either libswf or Ming to accomplish the same task - and it still is true of creating PDF files.
PHP provides integration with two very distinct PDF libraries - PDFlib, by Thomas Merz, and ClipPDF, by FastIO. Technically speaking, they are both quite similar, although ClipPDF does have the slight edge in that it is multi-threading safe and supports Unicode. Both versions require you to purchase a licence for commercial use, with ClipPDF costing $1000 per IP address and PDFlib costing $500 for a 1-CPU server.
Naturally we don't have the space to cover both libraries here, so we will be looking at PDFlib - by far the more popular of the two.