test1

$x = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];

function reverse($getArray) {
  if(!empty($getArray)) {
    $value  = array_pop($getArray);
    	 $sendElement = [$value => to_array($getArray)];
       return $sendElement;
     }
}

$x = reverse($x);

echo '<pre>';
print_r($x);
echo '</pre>';

test2

$data = [
    'parent.child.field' => 1,
    'parent.child.field2' => 2,
    'parent2.child.name' => 'test',
    'parent2.child2.name' => 'test',
    'parent2.child2.position' => 10,
    'parent3.child3.position' => 10,
];

function getResult1($getArray) {
    foreach($getArray as $i => $j) {
        $i = explode('.', $i);
        $result[$i[0]][$i[1]][$i[2]] = $j;
    }
    return $result;
}

$result1 = getResult1($data);
echo '<pre>';
print_r($result1);
echo '</pre><br>';

function getResult2($getArray) {
    foreach($getArray as $i1 => $j1) {
      foreach($j1 as $i2 => $j2) {
        foreach($j2 as $i3 => $j3) {
          $result["{$i1}.{$i2}.{$i3}"] = $j3;
        }
      }
    }
return $result;
}

$result2 = getResult2($result1);
echo '<pre>';
print_r($result2);
echo '</pre>';