<?php

class SmartArray {

public $data = [];

public function __construct(array $data = []) {
  $this->data = $data;
}

private static function search_in_array(array $data, $keys) {
  foreach ( $keys as $index => $key ) {
    if (isset($data[$key]))
      return $index;
  }
  return null;
}

public function __invoke(...$keys) {
  $data = $this->data;

  while ( !empty($keys) ) {
    $index = self::search_in_array($data, $keys);
    if ( is_null($index) ) return null;
    $data = $data[$keys[$index]];
    array_splice($keys, $index, 1);
  }

  return $data;
}

}

//USAGE

$data = [
 'a1' => [
   'b1' => 1,
   'b2' => 2
 ],
 'a2' => [
   'b1'=> 3,
   'b2' => [
     'c1' => 2
   ]
 ]
];

$data = new SmartArray($data);

var_dump($data('a1', 'b1'));
var_dump($data('b1', 'a1'));


var_dump($data('a2', 'b1'));
var_dump($data('b1', 'a2'));

var_dump($data('a2', 'b2', 'c1'));
var_dump($data('c1', 'b2', 'a2'));
var_dump($data('b2', 'c1', 'a2'));