how to to filter array inedes in php code example

Example 1: how can use filter in php7.2

Odd :
Array
(
    [a] => 1
    [c] => 3
    [e] => 5
)
Even:
Array
(
    [0] => 6
    [2] => 9
    [4] => 10
    [6] => 12
)

Example 2: filter value in array php return single value

?php
$data= [
    0 => [1, 'test1'],
    1 => [2, 'test2'],
    2 => [3, 'test3'],
];

$ids = array_map(function($item) {
    return $item[0];
}, $data);

var_dump($ids);