Export annotations in PHP

To extract data from PDF to FDF, then export FDF as XFDF

$doc = new PDFDoc($filename);

// Extract annotations to FDF.
// Optionally use e_both to extract both forms and annotations
$doc_fields = $doc->FDFExtract(PDFDoc::e_annots_only); //PDFDoc::e_forms_only

// Export annotations from FDF to XFDF.
$doc_fields->SaveAsXFDF($output_xfdf_filename);

// Optionally write XFDF to a string
$XFDF_string = $doc_fields->SaveAsXFDF();

PDF Form Fill and Form Data Extraction
Full sample code which illustrates basic FDF merge/extract functionality and full support for FDF (Forms Data Format)