Setting $_POST for filter_input_array(INPUT_POST) in phpunit test

Having some issues using PHPUnit to test my controllers.

Code I was working on so far was implementing $_POST or other request variables:

$_SERVER['REQUEST_METHOD'] = 'POST';
$_POST = array(
    'test' => true
);

Most of tests worked perfectly this way until I run into methods that take uses of filter_input_array function:

$_SERVER['REQUEST_METHOD'] = 'POST';
$_REQUEST = $_POST = $GLOBALS['_POST'] = array(
    'test' => true
);

// ....

var_dump(filter_input_array(INPUT_POST));

NULL

I'm not willing to remove filter_input functions from not mine code, but I'm unable to make them working in tests.

Versionings:
PHP 5.5.9-1ubuntu4.9 (cli) (built: Apr 17 2015 11:44:57)
Apache/2.4.7 (Ubuntu)
PHPUnit 4.6.6 by Sebastian Bergmann and contributors.

Any help will be appreciated.