效果是一样的。而__DIR__是PHP 5.3.0才引进的，所以一些已有的项目中更多看见dirname(__FILE__)。
__DIR__ : The directory of the file.
If used inside an include, the
directory of the included file is
returned. This is equivalent to
dirname(__FILE__). This directory
name does not have a trailing slash
unless it is the root directory.
(Added in PHP 5.3.0.) (source)
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