Php: what complexity [i.e O(N), O(1), ...] the the square brackets add function has? i.e $x[] = 'value'

Imagine you are adding several elements to an array:

$array[] = 'dog';
$array[] = 'cat';
$array[32] = 'lachupacabra';
$array[] = 'cow';

Do you know what index the 'cow' will have? No? Neither did I until I've found out that it's 33. This seems logical, php should have a reference to the last element added to array, and while adding element with square brackets syntax it takes previous element, takes it's index increments it by one and associates the incremented index with the new value. But stop, this might cause trouble if the incremented index was added earlier. Example:

$array[] = 'dog';
$array[33] = 'cat';
$array[32] = 'lachupacabra';
$array[] = 'cow';

Gues what, the index of 'cow' is 34. Does it mean that php tests all keys before adding another one with the square brackets syntax? This will have 开发者_Python百科O(N) complexity or is it storing something like $max_key value somewhere inside, the operation will have O(1) complexity but will use extra memory?

Question #1: What complexity the '[]' operation has?

Question #2: How the hell it is possible to get the index that the element was added at?

(Regarding Question#2 I'd prefer the O(1) operation if you don't mind)

EDIT

I need to implement the following method:

class X {

private $array;

public function AddItem($item) {
    $array[] = 'new value';

    return get_highest_index_with_o_1_comlexity($array);
}

public function RemoveItem($index) {
    unset($this->array[$index]);
}
}

To get the key of the last element in an array use:

end($array);
$key = key($array);

end positions the cursor on the last element of the array, key returns the key at the current cursor position.

PHP uses the (largest index + 1) as the next index when using []. For example, If you had $arr = array(42 => 'foo') then the next element assigned would have the key 43. Array inserts happen in constant time. It's extremely silly to worry about this.