It says it can populate a PDF with "pure PHP" however it executes against a dependency of pdftk (written in Java) which is compiled code that does not exist on every PHP server (and you won't find in most shared hosting environments):
You can do the same thing from reading the pdftk documentation.
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From Wikipedia, the free encyclopedia
PDFtk Developer(s) Sid Steward
Initial release July 14, 2004
Stable release 2.02 / July 24, 2013; 10 months ago
Written in Java
Operating system Cross-platform
Type PDF utility
PDFtk or The PDF Toolkit is an open source cross-platform tool for manipulating Portable Document Format (PDF) documents. PDFtk is a front end to the iText library (compiled to native code using GCJ), capable of splitting, merging, encrypting, decrypting, uncompressing, recompressing, and repairing PDFs. It can also be used to manipulate watermarks, metadata, and to fill PDF Forms with FDF Data (Forms Data Format) or XFDF Data (XML Form Data Format).