$item1 = 'password';
$item2 = 'firstname';

$this->addError(array($item1 => 'required'))
$this->addError(array($item2 => 'required'))

private function addError($error) {
    $this->_errors[] = $error;
}

public function error($item) {
    return array_search($item, $this->_errors);
}

Array
(
    [0] => Array
        (
            [password] => required
        )
    [1] => Array
        (
            [firstname] => required
        )
)

Array
(
    [password] => required
    [firstname] => required
)

private function addError($element, $error) {
    $this->_errors[$element] = $error;
}

$this->addError($item1, 'required');
$this->addError($item2, 'required');

private function addError($error) {
    $this->_errors[] = $error;
}

private function addError($error) {
    list($a,$b) = each($error);    //<--------- Add the list() here
    $this->_errors[$a] = $b; //<--------- Map those variables to your array
}