<?php
function recursive_array_search($needle, array $haystack) {
  foreach ($haystack as $key => $value) {
    if ($value === $needle) {
      return $key;
    }
    if (is_array($value)) {
      $chideKey = recursive_array_search($needle, $value);
      return [$key, $chideKey];
    }
  }
}
$data = [
    'a',
    'b',
    [
      'aa',
      'bb'
    ]
];
$ret = recursive_array_search('bb', $data);

var_dump($ret);