До изменения:array(3) {
  ["firstElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
  ["SecondElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
  ["ThirdElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
}

После изменения:array(3) {
  ["SecondElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
  ["ThirdElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
  ["firstElement"]=>
  array(2) {
    ["FirstKey"]=>
    string(3) "Foo"
    ["SecondKey"]=>
    string(3) "bar"
  }
}