Description:
------------
parse_ini_file cannot handle keys that start/end with an underscore.

Reproduce code:
---------------
// my.ini
test   = 1
_test  = 2
test_  = 3

[section]
test  = 1
_test = 2
test_ = 3

// ini.php
print_r(parse_ini_file('my.ini', true));

Expected result:
----------------
Array
(
    [test] => 1
    [_test] => 2
    [test_] => 3
    [section] => Array
        (
            [test] => 1
            [_test] => 2
            [test_] => 3
        )

)


Actual result:
--------------
Array
(
    [test] => 1
)