Happily, PDFlib and ClipPDF aren't all that far apart when it comes to using the two inside PHP: once you are familiar with PDFlib, it will probably be worth your while investigating whether ClipPDF is better suited to your needs.
One minor drawback to using ClipPDF is that the PHP documentation is much less complete for ClipPDF than it is for PDFlib, which would make learning it much trickier. Have a look over the documentation at http://uk.php.net/manual/en/ref.cpdf.php (or your local mirror) and see what you think.
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