<?php
$cache_timeout = 43200;
$file = 'today.xml';

function getCurrency($x){
    global $cache_timeout;
    global $file;
    if(!file_exists($file)){
        file_put_contents($file, file_get_contents('http://www.tcmb.gov.tr/kurlar/today.xml'));    
    }
    if(filemtime($file)+$cache_timeout<time()){
        file_put_contents($file, file_get_contents('http://www.tcmb.gov.tr/kurlar/today.xml'));    
    }
    $get = file_get_contents($file);
    $xml=simplexml_load_string($get);
    return $xml->xpath('Currency[@CurrencyCode="'.$x.'"]')[0]->BanknoteSelling;
}

echo getCurrency('USD');

?>