function putArrayToKey($key, array &$array)
{
    $array = array($key => count($array) > 0 ? $array : null);

    return $array;
}

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

$result = array();
foreach ($x as $item) {
    $this->putArrayToKey($item, $result);
}