$url = 'https://www.tcmb.gov.tr/kurlar/today.xml';
$ch = curl_init();
curl_setopt( $ch, CURLOPT_URL, $url );
curl_setopt( $ch, CURLOPT_RETURNTRANSFER, 1 );
$xml = curl_exec( $ch );
curl_close( $ch );

$xmlObj = simplexml_load_string( $xml );
//xml to json
$json = json_decode( json_encode( $xmlObj ), true );

$dolarKuruSatis = false;
$dolarKuruAlis = false;

if( $json && isset( $json['Currency'] ) ){
    foreach($json['Currency'] as $kur){
        if( $kur['@attributes']['CurrencyCode'] == 'USD' ){
            $dolarKuruSatis = $kur['ForexSelling'];
            $dolarKuruAlis = $kur['ForexBuying'];
            break;
        }
    }
}

echo 'Dolar Kuru Satış: ' . $dolarKuruSatis . '<br>';
echo 'Dolar Kuru Alış: ' . $dolarKuruAlis;