If you wish to create a pdf from php, pdflib will help you (as some others suggested).
Else, if you want to convert an HTML page to PDF via PHP, you'll find a little trouble outta here.. For 3 years I've been trying to do it as best as I can.
So, the options I know are:
DOMPDF : php class that wraps the html and builds the pdf. Works good, customizable (if you know php), based on pdflib, if I remember right it takes even some CSS. Bad news: slow when the html is big or complex.
HTML2PS: same as DOMPDF, but this one converts first to a .ps (ghostscript) file, then, to whatever format you need (pdf, jpg, png). For me is little better than dompdf, but has the same speed problem.. but, better compatibility with CSS.
Those two are php classes, but if you can install some software on the server, and access it throught passthru() or system(), give a look to these too:
wkhtmltopdf: based on webkit (safari's wrapper), is really fast and powerful.. seems like this is the best one (atm) for converting html pages to pdf on the fly; taking only 2 seconds for a 3 page xHTML document with CSS2. It is a recent project, anyway, the google.code page is often updated.
htmldoc : This one is a tank, it never really stops/crashes.. the project looks dead since 2007, but anyway if you don't need CSS compatibility this can be nice for you.