DateTime的方法鏈問題

Question

這工作：

$aWeekAfterDate = new DateTime(); 
$aWeekAfterDate->modify('+1 week'); 
$aWeekAfterDate->format('d.m.Y'); 

但這：

$aWeekAfterDate = new DateTime(); 
$aWeekAfterDate->modify('+1 week')->format('d.m.Y'); 

給了我這個錯誤：

Fatal error: Call to a member function format() on a non-object in ... on line ...

Fatal error (shutdown): Call to a member function format() on a non-object in ... on line ...

我已經看過了文檔中的DateTime::modify方法它的回報值是這樣的：

Returns the DateTime object for method chaining or FALSE on failure.

爲什麼方法鏈不起作用？

我使用PHP版本5.2.6。

Answer 1

更新日誌

Version Description 
5.3.0  Changed the return value on success from NULL to DateTime. 

於是久違的鏈接DateTime對象在PHP 5.3.0