(PHP 4, PHP 5 < 5.3.0, PECL fdf SVN)
fdf_save — Save a FDF document
fdf_documentThe FDF document handle, returned by fdf_create() , fdf_open() or fdf_open_string() .
filenameIf provided, the resulting FDF will be written in this parameter. Otherwise, this function will write the FDF to the default PHP output stream.
The doc says that an empty file name will cause PHP to output the FDF to 'stdout'. I had no success on this (with PHP 4.2.3 and fdftk v5): no string, an empty string and null all returned a message complaining about 'wrong parameter count'.
So I used the way (saving to a file and using passthru) given as an example on the page describing fdf_create(), and added a uniqid(rand(),1) to the filename to avoid clashes. This worked well.