I want to store data in new index as per my key in php [duplicate]

PHP Snippet 1:

$pieces = array_chunk($my_array, 6);

PHP Snippet 2:

<?php
$a = ["a","a","a","a","a","a","a","a"];

$b = array_chunk($a, 7);

echo"<pre>";print_r($b);

?>

PHP Snippet 3:

Array
(
    [0] => Array
        (
            [0] => a
            [1] => a
            [2] => a
            [3] => a
            [4] => a
            [5] => a
            [6] => a
        )

    [1] => Array
        (
            [0] => a
        )

)