Paylaşımlı hostinglerde genellikle allow_url_fopen fonksiyonu kapalı oluyor. İlgili siteye curl ile bağlanabilirsiniz.

<?php

$ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,'https://www.tcmb.gov.tr/kurlar/today.xml');
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER , 0);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER , 1);
    curl_setopt($ch, CURLOPT_HEADER         , 0);
    curl_setopt($ch, CURLOPT_TIMEOUT        , 30);
    curl_setopt($ch, CURLOPT_USERAGENT      , 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:32.0) Gecko/20100101 Firefox/32.0');

    $xml = curl_exec($ch);
    $kurcek = json_decode(json_encode((array)simplexml_load_string($xml)));
    curl_close($ch);