无法将多维数组转换为单个php数组

function flattenArray($array) {
    $result = array();
    foreach ($array as $element) {
        if (is_array($element)) {
            $result = array_merge($result, flattenArray($element));
        } else {
            $result[] = $element;
        }
    }
    return $result;
}

$multiDimensionalArray = array(
    array(1, 2, 3),
    array(4, 5, 6),
    array(7, 8, 9)
);

$flattenedArray = flattenArray($multiDimensionalArray);
print_r($flattenedArray);