Although this is a very old bug, it seems to me that the poster is 
incorrect; I suspect the problem is that the poster attempted to set the 
$_POST array during runtime (e.g. $_POST = array( ... ); ), and 
filter_input_array does not allow runtime modification $_POST, $_GET, 
etc.

Fast forward to 2012, this is still wrong IMHO

Possible bugs:
1) documentation problem; the return value is defined as an array or false in case of failure - when the respective INPUT_XXX is not populated, it returns null; clearly not expected according to the documentation.
2) wrong behaviour in the code; if INPUT_XXX is not populated, I would expect the return value to be this: array_fill_keys(array_keys($definition), null)

Changing the behavior of filter_input_array() for cases where
INPUT_* is not set would cause a BC break, and in my opinion would
therefore require the RFC process[1], since filter_input_array()
works this way since its introduction[2].  Thus, I'm changing to
doc bug.

It is also noteworthy to mention that filter_input_array() returns
FALSE, if the input array is not populated, and the
FILTER_NULL_ON_FAILURE flag is set in the $definition array[3].

Finally, indeed the example is seriously wrong.

[1] <https://wiki.php.net/rfc/howto>
[2] <https://3v4l.org/8qjcU>
[3] <https://3v4l.org/N9uDe>

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.This bug has b