(i filed a bug report/feature request for this on bugs.php.net and assigned it to myself email@example.com)
The default behavior for the FDF Toolkit is to return an FDFErcNoValue for the FDFGetValue when the field exists but has no value.
Whether or not this is truly an error is debateable. It seems to become an issue in documents created by Acrobat 5 when optional fields are included in a form with required fields since the default behavior seems to be to populate the HTTP_FDF_DATA with the results of the FDFNextFieldName enumerator, which would include the fields which have no value.
This would not be a problem except that the PHP function fdf_get_value() does not specifically test for the FDFErcNoValue condition but, instead, tests for the more general FDFErcOK. If this value is not the result of the error code, the system issues warnings (the display of which could be turned off), but the broader question is, should this be a warning condition at all?
Put another way, should a return of FDFErcNoValue for FDFGetValue be considered an event worth generating a warning?