Note that although you can provide a default filter for the entire input array there is no way to provide a flag for that filter without building the entire definition array yourself.

So here is a small function that can alleviate this hassle!

<?php
function filter_input_array_with_default_flags($type, $filter, $flags, $add_empty = true) {
    $loopThrough = array();
    switch ($type) {
        case INPUT_GET : $loopThrough = $_GET; break;
        case INPUT_POST : $loopThrough = $_POST; break;
        case INPUT_COOKIE : $loopThrough = $_COOKIE; break;
        case INPUT_SERVER : $loopThrough = $_SERVER; break;
        case INPUT_ENV : $loopThrough = $_ENV; break;
    }
   
    $args = array();
    foreach ($loopThrough as $key=>$value) {
        $args[$key] = array('filter'=>$filter, 'flags'=>$flags);
    }
    
    return filter_input_array($type, $args, $add_empty);
}
?>

[New Version]
This function is very useful for filtering complicated array structure.
Also, Some integer bitmasks and invalid UTF-8 sequence detection are available.

Code:
<?php
/**
 * @param  integer $type    Constant like INPUT_XXX.
 * @param  array   $default Default structure of the specified super global var.
 *                          Following bitmasks are available:
 *  + FILTER_STRUCT_FORCE_ARRAY - Force 1 dimensional array.
 *  + FILTER_STRUCT_TRIM        - Trim by ASCII control chars.
 *  + FILTER_STRUCT_FULL_TRIM   - Trim by ASCII control chars,
 *                                full-width and no-break space.
 * @return array            The value of the filtered super global var.
 */
define('FILTER_STRUCT_FORCE_ARRAY', 1);
define('FILTER_STRUCT_TRIM', 2);
define('FILTER_STRUCT_FULL_TRIM', 4);
function filter_struct_utf8($type, array $default) {
    static $func = __FUNCTION__;
    static $trim = "[\\x0-\x20\x7f]";
    static $ftrim = "[\\x0-\x20\x7f\xc2\xa0\xe3\x80\x80]";
    static $recursive_static = false;
    if (!$recursive = $recursive_static) {
        $types = array(
            INPUT_GET => $_GET,
            INPUT_POST => $_POST,
            INPUT_COOKIE => $_COOKIE,
            INPUT_REQUEST => $_REQUEST,
        );
        if (!isset($types[(int)$type])) {
            throw new LogicException('unknown super global var type');
        }
        $var = $types[(int)$type];
        $recursive_static = true;
    } else {
        $var = $type;
    }
    $ret = array();
    foreach ($default as $key => $value) {
        if ($is_int = is_int($value)) {
            if (!($value | (
                FILTER_STRUCT_FORCE_ARRAY |
                FILTER_STRUCT_FULL_TRIM | 
                FILTER_STRUCT_TRIM
            ))) {
                $recursive_static = false;
                throw new LogicException('unknown bitmask');
            }
            if ($value & FILTER_STRUCT_FORCE_ARRAY) {
                $tmp = array();
                if (isset($var[$key])) {
                    foreach ((array)$var[$key] as $k => $v) {
                        if (!preg_match('//u', $k)){
                            continue;
                        }
                        $value &= FILTER_STRUCT_FULL_TRIM | FILTER_STRUCT_TRIM;
                        $tmp += array($k => $value ? $value : '');
                    }
                }
                $value = $tmp;
            }
        }
        if ($isset = isset($var[$key]) and is_array($value)) {
            $ret[$key] = $func($var[$key], $value);
        } elseif (!$isset || is_array($var[$key])) {
            $ret[$key] = null;
        } elseif ($is_int && $value & FILTER_STRUCT_FULL_TRIM) {
            $ret[$key] = preg_replace("/\A{$ftrim}++|{$ftrim}++\z/u", '', $var[$key]);
        } elseif ($is_int && $value & FILTER_STRUCT_TRIM) {
            $ret[$key] = preg_replace("/\A{$trim}++|{$trim}++\z/u", '', $var[$key]);
        } else {
            $ret[$key] = preg_replace('//u', '', $var[$key]);
        }
        if ($ret[$key] === null) {
            $ret[$key] = $is_int ? '' : $value;
        }
    }
    if (!$recursive) {
        $recursive_static = false;
    }
    return $ret;
}
?>

This function is very useful for filtering complicated array structure.

Code:
<?php
function filter_request($var, $default_structure) {

    $ret = array();

    foreach ($default_structure as $key => $value) {
        if (!isset($var[$key])) {
            $ret[$key] = $value;
        } elseif (is_array($value)) {
            $ret[$key] = filter_request($var[$key], $value);
        } elseif (is_array($var[$key])) {
            $ret[$key] = $value;
        } else {
            $ret[$key] = $var[$key];
        }
    }

    return $ret;

}
?>

Sample Usage:
<?php
$_GET['a']['wrong_structure'] = 'foo';
$_GET['b']['c'] = 'CORRECT';
$_GET['b']['d']['wrong_structure'] = 'bar';
$_GET['unneeded_item'] = 'baz';

var_dump(filter_request($_GET, array(
    'a' => 'DEFAULT',
    'b' => array(
        'c' => 'DEFAULT',
        'd' => 'DEFAULT',
    ),
)));
?>

Sample Result:
array(2) {
  ["a"]=>
  string(21) "DEFAULT"
  ["b"]=>
  array(2) {
    ["c"]=>
    string(12) "CORRECT"
    ["d"]=>
    string(21) "DEFAULT"
  }
}

Beware: if none of the arguments is set, this function returns NULL, not an array of NULL values.

/* No POST vars set in request
$_POST = array();
*/

$args = array('some_post_var' => FILTER_VALIDATE_INT);
$myinputs = filter_input_array(INPUT_POST, $args);
var_dump($myinputs);

Expected Output: array(1) { ["some_post_var"]=> NULL } 

Actual Output: NULL

Looks like filter_input_array isn't aware of changes to the input arrays that were made before calling filter_input_array. Instead, it always looks at the originally submitted input arrays.

So this will not work:

$_POST['my_float_field'] = str_replace(',','.',$_POST['my_float_field']);
$args = array('my_float_field',FILTER_VALIDATE_FLOAT);
$result = filter_input_array(INPUT_POST, $args);

[New Version]

Example Usage:
<?php
$_GET['A']['a'] = '  CORRECT(including some spaces)    ';
$_GET['A']['b'] = '  CORRECT(including some spaces)    ';
$_GET['A']['c'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['A']['d']['invalid_structure'] = 'INVALID';

$_GET['B']['a'] = '  CORRECT(including some spaces)    ';
$_GET['B']['b'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['B']['c']['invalid_structure'] = 'INVALID';
$_GET['B']["Invalid UTF-8 sequence: \xe3\xe3\xe3"] = 'INVALID';

$_GET['C']['a'] = '  CORRECT(including some spaces)    ';
$_GET['C']['b'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['C']['c']['invalid_structure'] = 'INVALID';
$_GET['C']["Invalid UTF-8 sequence: \xe3\xe3\xe3"] = 'INVALID';

$_GET['unneeded_item'] = 'UNNEEDED';

var_dump(filter_struct_utf8(INPUT_GET, array(
    'A' => array(
        'a' => '',
        'b' => FILTER_STRUCT_TRIM,
        'c' => '',
        'd' => '',
    ),
    'B' => FILTER_STRUCT_FORCE_ARRAY,
    'C' => FILTER_STRUCT_FORCE_ARRAY | FILTER_STRUCT_TRIM,
)));
?>

Example Result:
array(3) {
  ["A"]=>
  array(4) {
    ["a"]=>
    string(36) "  CORRECT(including some spaces)    "
    ["b"]=>
    string(30) "CORRECT(including some spaces)"
    ["c"]=>
    string(0) ""
    ["d"]=>
    string(0) ""
  }
  ["B"]=>
  array(3) {
    ["a"]=>
    string(36) "  CORRECT(including some spaces)    "
    ["b"]=>
    string(0) ""
    ["c"]=>
    string(0) ""
  }
  ["C"]=>
  array(3) {
    ["a"]=>
    string(30) "CORRECT(including some spaces)"
    ["b"]=>
    string(0) ""
    ["c"]=>
    string(0) ""
  }
}

While filtering input arrays, be careful of what flags you set besides FILTER_REQUIRE_ARRAY. For example, setting the flags like so:

<?php
$filter = array(
'myInputArr' => array('filter' => FILTER_SANITIZE_STRING,
                      'flags' => array('FILTER_FLAG_STRIP_LOW', 'FILTER_REQUIRE_ARRAY'))
);

$form_inputs = filter_input_array(INPUT_POST, $filter);
?>

.. will result in a blank $form_inputs['myInputArr'] regardless of what $_POST['myInputArr'] contains.

If you are trying to handling multiple form inputs with same name, then you must assign the `'flags'  => FILTER_REQUIRE_ARRAY` to the definitions entry.

Example, you have a html form as such:
<form>
 <input name="t1[]" value="Some string One" />
 <input name="t1[]" value="Another String Two" />
</form>

Your definitions array will look a little like this:
$args = array(
  't1'    => array(
      'name' => 't1',
      'filter' => FILTER_SANITIZE_STRING,
      'flags'  => FILTER_REQUIRE_ARRAY)
);

Here's an extended function that allows you to keep also the unfiltered items/args from the request, while you also apply validation to some of them:

<?php

$validationRules = [
    'foo' => [
        'filter' => FILTER_VALIDATE_REGEXP,
        'options' => ['regexp' => '/^(bar|baz)$/i']
    ]
];

$request = filter_input_array_keep_unfiltered_args(INPUT_POST, $validationRules);

var_dump($request);

function filter_input_array_keep_unfiltered_args($type, $filters, $addEmpty = true)
{
    $rawRequest = filter_input_array($type);

    $validationRules = [];
    foreach ($rawRequest as $key => $value) {
        $validationRules[$key] = isset($filters[$key]) ? $filters[$key] : ['filter' => FILTER_DEFAULT];
    }

    return filter_input_array($type, $validationRules, $addEmpty);
}

?>