filter_input_array on a multidimensional array

I have the following array from a result from var_export( $post_meta ); from $post_meta = get_post_meta( 80 );

array (
  '_edit_last' => 
  array (
    0 => '1',
  ),
  '_edit_lock' => 
  array (
    0 => '1451326767:1',
  ),
  '_sidebar' => 
  array (
    0 => 'Kies Sidebar',
  ),
  '_wp_page_template' => 
  array (
    0 => 'page-pop.php',
  ),
  'custom_sidebar_per_page' => 
  array (
    0 => 'default',
  ),
  '_cat_id' => 
  array (
    0 => '21',
  ),
  '_order_by' => 
  array (
    0 => 'date',
  ),
  '_asc' => 
  array (
    0 => 'DESC',
  ),
  '_post_count' => 
  array (
    0 => '5',
  ),
  '_days' => 
  array (
    0 => '0',
  ),
  '_custom_sidebar_per_page' => 
  array (
    0 => 'default',
  ),
)

Now I need to filter a few of those values if they exist so I can safely use them. I do the following

$args = [
    '_cat_id' => [
        0 => [
            'filter' => FILTER_VALIDATE_INT,
            'default' => 1
        ] 
    ],         
    '_page_title' => [
        0 => FILTER_SANITIZE_STRING,
    ],
    '_posts_title' => [
        0 => FILTER_SANITIZE_STRING,
    ],
    '_order_by' => [
        0 => [
            'filter'  => FILTER_SANITIZE_STRING,
            'default' => 'date'
        ]
    ],
    '_asc' => [
        0 => [
            'filter'  => FILTER_SANITIZE_STRING,
            'default' => 'DESC'
        ]
    ],
    '_post_count' => [
        0 => [
            'filter'  => FILTER_VALIDATE_INT,
            'default' => get_option( 'posts_per_page' )
        ]  
    ]
];  
$meta = filter_var_array( $post_meta, $args );

but I get the following results from var_export( $meta )

array (
  '_cat_id' => false,
  '_page_title' => NULL,
  '_posts_title' => NULL,
  '_order_by' => false,
  '_asc' => false,
  '_post_count' => false,
)

Something like _cat_id should return something like

'_cat_id' => 
  array (
    0 => 21,
  ),

in the resultant array.

Any ideas on how to use filter_var_array on a multidimensional array

You could first "unnest" your array $post_meta, by popping the element out of each sub-array with the use of array_map and array_pop:

$post_meta_flat = array_map('array_pop', $post_meta);

This array $post_meta_flat will look like this:

array (
  '_edit_last' => '1',
  '_edit_lock' => '1451326767:1',
  '_sidebar' => 'Kies Sidebar',
  '_wp_page_template' => 'page-pop.php',
  'custom_sidebar_per_page' => 'default',
  '_cat_id' => '21',
  '_order_by' => 'date',
  '_asc' => 'DESC',
  '_post_count' => '5',
  '_days' => '0',
  '_custom_sidebar_per_page' => 'default',
)

And now this should work:

$meta = filter_var_array( $post_meta_flat, $args );

You can of course do both in a one-liner:

$meta = filter_var_array( array_map('array_pop', $post_meta), $args );