a little performance improvment: replace dirname(__FILE__) with __DIR__

[nigro.simone]

A little performance improvment is replace

dirname(__FILE__)

with

__DIR__

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

For example, the two following lines :

<?php
var_dump(dirname(__FILE__));
var_dump(__DIR__);

Will both give the same output :

string '/htdocs/wordpress' (length=17)

But, there are at least two differences :

1) _DIR_ only exists with PHP >= 5.3
2) _DIR_ is evaluated at compile-time, while dirname(_FILE_) means a function-call and is evaluated at execution-time, so, _DIR_ is faster.

[ocean90]

Duplicate of #32274.