Is there any difference between __DIR__ and dirname(__FILE__) in PHP?

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Their result is exactly the same ; so, no difference on that.

For example, the two following lines :

var_dump(dirname(__FILE__));
var_dump(__DIR__);

Will both give the same output :

string '/home/squale/developpement/tests/temp' (length=37)

But, there are at least two differences :

  • __DIR__ only exists with PHP >= 5.3
    • which is why dirname(__FILE__) is more widely used
  • __DIR__ is evaluated at compile-time, while dirname(__FILE__) means a function-call and is evaluated at execution-time
    • so, __DIR__ is (or, should be) faster.

As, as a reference, see the Magic constants section of the manual (quoting) :

__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.)

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