Description:
------------
is_writable() has problems to handle certain proper unix 
paths like "/some/dir/../dir/file.suffix"

Reproduce code:
---------------
<?php 

$basedir = dirname(__FILE__);
require_once($basedir.'/../dir/blah.php'); /* this works fine. */

if (!is_writable($basedir.'/../dir/blah.php')) 
{
	echo '1 - blah.php not writeable.';
}
else 
{
    echo '1 - OK';
}

if (!is_writable('../dir/blah.php')) 
{
    echo '2 - blah.php not writeable.';
}
else 
{
    echo '2 - OK';
}

?>

Expected result:
----------------
1 - OK
2 - OK

Actual result:
--------------
1 - blah.php not writeable.
2 - OK