<?php

$url_yt = "https://www.googleapis.com/youtube/v3/channels?part=statistics&id=CHANNELID&key=AIzaSyDLfPQI-rIjdZrd-9hYWOCahgSkZ0J8lgo";

$urlParts = parse_url($url_yt);
if($urlParts['query']) {
    parse_str($urlParts['query'], $params);

    if($params['id']) {
        $params['id'] = 'hello world'; // $arResult['DISPLAY_PROPERTIES']['QUERY']['VALUE']
        $urlParts['query'] = http_build_query($params);
    }
}

echo unparse_url($urlParts);

/**
* @link http://php.net/manual/en/function.parse-url.php#106731
*/
function unparse_url($parsed_url) {
    $scheme   = isset($parsed_url['scheme']) ? $parsed_url['scheme'] . '://' : '';
    $host     = isset($parsed_url['host']) ? $parsed_url['host'] : '';
    $port     = isset($parsed_url['port']) ? ':' . $parsed_url['port'] : '';
    $user     = isset($parsed_url['user']) ? $parsed_url['user'] : '';
    $pass     = isset($parsed_url['pass']) ? ':' . $parsed_url['pass']  : '';
    $pass     = ($user || $pass) ? "$pass@" : '';
    $path     = isset($parsed_url['path']) ? $parsed_url['path'] : '';
    $query    = isset($parsed_url['query']) ? '?' . $parsed_url['query'] : '';
    $fragment = isset($parsed_url['fragment']) ? '#' . $parsed_url['fragment'] : '';
    return "$scheme$user$pass$host$port$path$query$fragment";
}