Description:
------------
When I create many named groups in pattern like

\(?'group0'foo)|(?'group1'bar)|…\

then the resulting $matches does not show 'group254'.
What was supposed to be in `$matches['group254']` is in `$matches['group255']`, and so on.

the lack repeats every ~255 named groups and the offset is getting worse and worse.

I tested it on php 7 and php 5.5, locally and in online-php-sandbox.

Test script:
---------------
<?php
$pattern = [];
for ($i = 0; $i < 300; $i++) {
    $pattern[] = "(?'group{$i}'{$i}$)";
}

$fullPattern = '/' . implode('|', $pattern) . '/uix';

preg_match($fullPattern, '290', $matches);
var_dump(isset($matches['group253']));
var_dump(isset($matches['group254']));
var_dump(isset($matches['group255']));

var_dump($matches['group290']);
var_dump($matches['group291']);

Expected result:
----------------
bool(true)
bool(true)
bool(true)
string(3) "290"
string(0) ""

Actual result:
--------------
bool(true)
bool(false)
bool(true)
string(0) ""
string(3) "290"