Не пишите 'xml' большими буквами:

<?php

/**
 * @param $url
 * @return array
 */

function curlGet($url)
{

    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'GET');

    curl_setopt($ch, CURLOPT_ENCODING, 'gzip, deflate');

    $headers = array();
    $headers[] = 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9';
    $headers[] = 'Accept-Language: ru,en;q=0.9,ru-RU;q=0.8,en-US;q=0.7';
    $headers[] = 'Cache-Control: no-cache';
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

    $result = array('st' => 'OK', 'data' => curl_exec($ch));
    if (curl_errno($ch)) {
        $result['st'] = 'Error';
        $result['ErrorMsg'] = curl_error($ch);
    }
    curl_close($ch);
    return $result;
}

$xmlData = curlGet('http://www.cbr.ru/scripts/XML_daily.asp?date_req=25.08.2022');
var_dump($xmlData);

$client->setOption('format', 'xml');

{
	"templates": [{
		"title": "Replace content template",
		"description": "This template includes a button, and the label of the button will be replaced on insert.",
		"content": "<button class=\"button\"><span class=\"content-replacement\">[selected content will insert here]</span></button>"
	}],
	"template_selected_content_classes": "content-replacement"
}