Description:
------------
When the SORT_NATURAL | SORT_FLAG_CASE flags are passed to array_multisort(), it 
sorts strings beginning with the letter "a" above strings beginning with "_", even 
though "_" has a lower ordinal value.

natsort() places "_" before "a" as I would expect, and array_multisort() places 
"_" before "a" when given only the SORT_NATURAL flag, without ORing it with 
SORT_FLAG_CASE.

See the test script for details.

Test script:
---------------
<?php
$keys   = ["_", "a"];
$values = ["first", "second"];

// Results in incorrect order.
array_multisort($keys, SORT_NATURAL | SORT_FLAG_CASE, $values, SORT_ASC);
print_r($keys);
print_r($values);

// Results in correct order.
natsort($keys);
print_r($keys);

// Results in correct order.
array_multisort($keys, SORT_NATURAL, $values, SORT_ASC);
print_r($keys);
?>

Expected result:
----------------
I expect "_" to be sorted before "a", just as natsort() does.

Actual result:
--------------
"a" is sorted before "_".