ADV test

solutions.php

<?php
/* test1.php */
$x = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];;
$x = array_reduce(
    $x,
    function ($t, $v) {
        $prev = $t;
        $t = [];
        $t[$v] = $prev;

        return $t;
    },
    ''
);
echo'<pre>',print_r($x),'</pre>';

/* test2.php */
$data1 = [
    'parent.child.field' => 1,
    'parent.child.field2' => 2,
    'parent2.child.name' => 'test',
    'parent2.child2.name' => 'test',
    'parent2.child2.position' => 10,
    'parent3.child3.position' => 10,
];
$new_arr = [];
foreach ($data1 as $k => $v) {
    $parts = explode('.', $k);
    $end = sizeof($parts) - 1;
    $cur_item = &$new_arr;
    foreach ($parts as $part_key => $part_index) {
        if (!isset($cur_item[$part_index])) {
            $cur_item[$part_index] = [];
        }

        $cur_item = &$cur_item[$part_index];

        if ($part_key == $end) {
            $cur_item = $v;
        }
    }

}
echo'<pre>',print_r($new_arr),'</pre>';

$data = [
    'parent' => [
        'child' => [
            'field' => 1,
            'field2' => 2,
        ]
    ],
    'parent2' => [
        'child' => [
            'name' => 'test'
        ],
        'child2' => [
            'name' => 'test',
            'position' => 10
        ]
    ],
    'parent3' => [
        'child3' => [
            'position' => 10
        ]
    ],
];
$new_data = [];
foreach ($data as $k => $item) {
    $path = $k;
    restorePath($new_data, $path, $item);
}

function restorePath(&$new_data, $path, $el)
{
    if (is_array($el)) {
        foreach ($el as $k => $v) {
            restorePath($new_data, $path . '.' . $k, $v);
        }
    } else {
        $new_data[$path] = $el;
    }
}

echo'<pre>',print_r($new_data),'</pre>';

/**
 * И да, на стековерфлоу я закрыл несколько дубликатов подобных вопросов)
 */