$data = $service->get_data(); // get_data()返回的数据可能有上万条。
$data = & $service->get_data();
// 看了php作者不是太赞同这样用（from programming php, Rasmus Lerdorf），
// 谁能帮我解释下。 谢谢。
This technique is sometimes used to return large string or array values efficiently from a function. However, PHP’s copy-on-write/shallow-copy mechanism usually means that returning a reference from a function is not necessary. There is no point in returning a reference to some large piece of data unless you know you are likely to change that data. The drawback of returning the reference is that it is slower than returning the value and relying on the shallow-copy mechanism to ensure that a copy of that data is not made unless it is changed.