PHP: What value array key needs to have to cause next numeric key assignment?

Question

In other words, what value can be assigned to $key so that the following two lines create exactly the same result:

$arr[] = $value;
$arr[$key] = $value;

The task I need to solve looks like the following: I need to add a new value to an array using either the key if specified, or next available numeric key if key is not specified:

protected $_arr;

...
public function addValue($value, $key = NULL) {

    $this->_arr[$key] = $value;

}

Working solutions is like this:

public function addValue($value, $key = NULL) {

    if($key === NULL) $this->_arr[] = $value;
    else $this->_arr[$key] = $value;
}

I suspect there is no such value for the key as I tried obvious NULL and '' (empty string), and neither brings desired result.