php中的Alexa api不能正常工作

I have been trying the following code in PHP to extract the alexa website ranking

<?php

 $url="http://stackoverflow.com/";
 $xml = simplexml_load_file('http://data.alexa.com/data?cli=10&dat=snbamz&url='.$url);
 $rank=isset($xml->SD[1]->POPULARITY)?$xml->SD[1]->POPULARITY->attributes()->TEXT:0;
 $web=(string)$xml->SD[0]->attributes()->HOST;
 echo $web." has Alexa Rank ".$rank;
?>

I dont know what is wrong. Here is the output

SD[1]->POPULARITY)?$xml->SD[1]->POPULARITY->attributes()->TEXT:0; $web=(string)$xml->SD[0]->attributes()->HOST; echo $web." has Alexa Rank ".$rank; ?>

Am I missing some library? or what?