postinst is in php-fpdf 3:1.7.dfsg-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 | #!/bin/sh -e
if [ "$1" = configure ] && [ ! -L /usr/share/fpdf ] && dpkg --compare-versions "$2" lt-nl 1.53.dfsg-3; then
	rm -rf /usr/share/fpdf
	ln -s php/fpdf /usr/share/fpdf
fi
 |