$arr                = array();
$arr['a']           = array('FOO' => 'a',   'BAR' => 'a2');
$arr['a']['b']      = array('FOO' => 'ab',  'BAR' => 'ab2');
$arr['a']['b']['c'] = array('FOO' => 'abc', 'BAR' => 'abc2');

array(1) {
  ["a"]=>
  array(3) {
    ["FOO"]=>
    string(1) "a"
    ["BAR"]=>
    string(2) "a2"
    ["b"]=>
    array(3) {
      ["FOO"]=>
      string(2) "ab"
      ["BAR"]=>
      string(3) "ab2"
      ["c"]=>
      array(2) {
        ["FOO"]=>
        string(3) "abc"
        ["BAR"]=>
        string(4) "abc2"
      }
    }
  }
}