This example would cause a browser to open 'a.pdf' and pop up an alert window AFTER setting a field value:

<?php
header('Content-type: application/vnd.fdf');

$fdf = fdf_create();

fdf_set_file($fdf, 'http://www.example.com/path/to/a.pdf');
fdf_set_value($fdf, 'field1', 'my value');
fdf_set_on_import_javascript($fdf, 'app.alert("executing javascript");', false);

fdf_save($fdf);
fdf_close($fdf);
?>

Or, to execute the javascript BEFORE setting the field value, just change false to true:

<?php
header('Content-type: application/vnd.fdf');

$fdf = fdf_create();

fdf_set_file($fdf, 'http://www.example.com/path/to/a.pdf');
fdf_set_value($fdf, 'field1', 'my value');
fdf_set_on_import_javascript($fdf, 'app.alert("executing javascript");', true);

fdf_save($fdf);
fdf_close($fdf);
?>