Description:
------------
A file that require()s a config file located in dirname(__FILE__)
and outputs a constant is hard-linked in multiple sites hosted on the
same server. The constant for the site from which the file was requested the
first time gets cached and is returned in subsequent requests regardless
of the site.

It works fine if the file is *unhardlinked*. It also works if
dirname(__FILE__) is removed or just replaced with './'.

Reproduce code:
---------------
foo.example.com and bar.example.com have the same copies of test.php
and config.php.

# sha1sum /www/*/{test,config}.php
a0af22d58a7a7a5fd11547cd304cc08968c13d5c  /www/bar.example.com/test.php
a0af22d58a7a7a5fd11547cd304cc08968c13d5c  /www/foo.example.com/test.php
4494ec973dfed496e4b28f1ae6e4e2224d10a66c  /www/bar.example.com/config.php
4494ec973dfed496e4b28f1ae6e4e2224d10a66c  /www/foo.example.com/config.php

but test.php is hard-linked

# ls -il /www/*/{test,config}.php | awk '{print $1, $10}' | sort -rn
5640819 /www/foo.example.com/test.php
5640819 /www/bar.example.com/test.php
4792040 /www/bar.example.com/config.php
4792039 /www/foo.example.com/config.php

# cat /www/foo.example.com/test.php
<?php
require(dirname(__FILE__) . '/config.php');
echo ABSPATH;
?>

# cat /www/foo.example.com/config.php
<?php
define('ABSPATH', dirname(__FILE__).'/');
?>

Expected result:
----------------
"echo ABSPATH;" should return the ABSPATH for foo if foo.example.com is requested and the ABSPATH for bar if bar.example.com is requested.

Actual result:
--------------
# service httpd restart
Stopping httpd:                                            [  OK  ]
Starting httpd:                                            [  OK  ]

# curl -H 'Host: foo.example.com' http://127.0.0.1/test.php
/www/foo.example.com/

# curl -H 'Host: bar.example.com' http://127.0.0.1/test.php
/www/foo.example.com/

# service httpd restart
Stopping httpd:                                            [  OK  ]
Starting httpd:                                            [  OK  ]

# curl -H 'Host: bar.example.com' http://127.0.0.1/test.php
/www/bar.example.com/

# curl -H 'Host: foo.example.com' http://127.0.0.1/test.php
/www/bar.example.com/