Description:
------------
parse_ini_file() is not parsing constants that begin with an underscore.

CentOS 5 / PHP 5.3.1 from source

Note: This did work as expected in PHP 5.1.6

Reproduce code:
---------------
##### ini.ini #####
[section]
key1 = "no error"
key2buggy = __STRING
key3buggy = __STRING" stays mainly on the plain."

##### test.php #####
<?php
define('__STRING', 'The rain in Spain');
$arr = parse_ini_file('./ini.ini');
print_r($arr);

Expected result:
----------------
Array
(
    [section] => Array
        (
            [key1] => no error
            [key2buggy] => The rain in Spain
            [key3buggy] => The rain in Spain stays mainly on the plain.
        )

)

Actual result:
--------------
Array
(
    [section] => Array
        (
            [key1] => no error
            [key2buggy] => __STRING
            [key3buggy] => __STRING stays mainly on the plain.
        )

)