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.
fdf_save_string