http生成查询的意外输出(&=>ms)

$params = [
    'a' => 'x',
    'b' => 'y'
];
$url = http_build_query(['params' => $params]);
echo urldecode($url);

我期待这个结果:

params[a]=x&params[b]=y

但我得到的却是:

params[a]=x¶ms[b]=y

我也尝试过:

http_build_query(['params' => $params], '', '&');
http_build_query(['params' => $params], null, '&', PHP_QUERY_RFC3986);
http_build_query(['params' => $params], null, ini_get('arg_separator.output'), PHP_QUERY_RFC3986);

以及:

$params = [
    'params[a]' => 'x',
    'params[b]' => 'y'
];

但我总是得到同样的结果。我做错什么了吗?是我的服务器吗?

遵循@Sammitch的建议:

var_dump($url, bin2hex($url), urldecode($url), bin2hex(urldecode($url)));

输出:

string(31) "params%5Ba%5D=x&params%5Bb%5D=y"
string(62) "706172616d73253542612535443d7826706172616d73253542622535443d79"
string(23) "params[a]=x&params[b]=y"
string(46) "706172616d735b615d3d7826706172616d735b625d3d79"