$array = ['key1'=>['key2'=>'val']];

$keys_string = 'key1.key2';

$keys = explode(".", $keys_string);

$array[$keys[0]][$keys[1]] = 'val'

$keys = explode(".", $keys_string);
foreach($item as $keys) {
   $val = $array[($item];
}
$val = 'test';