One of frustrating features of PHP 5 type hinting is handling of null in type hinted functions. In strict mode, this will raise a notice, so we can use type hinting only if the function accepts no mixed arguments.

In this case, we do usually write a docblock @param to declare the supported parameters:

/**
 * @param $arg \DateTime|null
 */
function acceptsDateTimeOrNull($arg) {

}

It would be useful if NetBeans would enable code completion based on the @param docblock, at least when there is only one class and |null.