From PDFLib's manual:
"When the optional PDI (PDF import) library is attached to PDFlib, pages from existing PDF documents can be imported. PDI contains a parser for the PDF file format, and prepares pages from existing PDF documents for easy use with PDFlib.
Conceptually, imported PDF pages are treated similarly to imported raster images such as TIFF or PNG: you open a PDF document, choose a page to import, and place it on an output page, applying any of PDFlib’s transformation functions for translating, scaling, rotating, or skewing the imported page."
The manual can be downloaded along with PDFLib at http://www.pdflib.com/products/pdflib/download/index.html
(PHP 4 >= 4.0.5, PECL pdflib >= 1.0.0)
PDF_open_pdi — Open PDF file [deprecated]
Opens a disk-based or virtual PDF document and prepares it for later use.
This function is deprecated since PDFlib version 7, use PDF_open_pdi_document() instead.
The function returns the intval -1 on failure.