The core class for importing pages from existing PDF documents. FPDI is an extended class of FPDF_TPL which adds a template feature to FPDF.
As of version 1.2.1 FPDI is also compatible with TCPDF!
Please notice that as of version 1.2.1 you're responisble to include the class to extend (FPDF or TCPDF) before including FPDI.
FPDI transforms PDF pages into the template structure offered by FPDF_TPL.
You can use a source page by passing the id, returned from FPDI::importPage(), to the FPDF_TPL::useTemplate() method extended from FPDF_TPL. Of course, the id can be used to fetch the dimensions of a page by passing it to the FPDF_TPL::getTemplateSize() method as well.