用PHP和xpath解析XML -正确获取路径

我试图用php SimpleXML和xpath解析XML文件，并有问题获得正确的路径和访问属性值。谢谢你的帮助。

这是我的xml文件:-

<message:MessageGroup xmlns="http://mynamespace.com">
<Book>
<BookKey>
<Value concept="TITLE" value="Gone Girl"/>
<Value concept="AUTHOR" value="Gillian Flynn"/>
</BookKey>
<Sales>
<Month>Jan</Month>
<Number value="20"/>
</Sales>
<Sales>
<Month>Feb</Month>
<Number value="15"/>
</Sales>
<Sales>
<Month>Mar</Month>
<Number value="30"/>
</Sales>
</Book>
<Book>
<BookKey>
<Value concept="TITLE" value="Inferno"/>
<Value concept="AUTHOR" value="Dan Brown"/>
</BookKey>
<Sales>
<Month>Jan</Month>
<Number value="10"/>
</Sales>
<Sales>
<Month>Feb</Month>
<Number value="15"/>
</Sales>
<Sales>
<Month>Mar</Month>
<Number value="3"/>
</Sales>
</Book>
</message:MessageGroup>

我需要解析它以获得以下输出:-

Gone Girl,Gillian Flynn,Jan,20
Gone Girl,Gillian Flynn,Feb,15
Gone Girl,Gillian Flynn,Mar30
Inferno,Dan Brown,Jan,10
Inferno,Dan Brown,Feb,15
Inferno,Dan Brown,Mar,3

我写了下面的代码:-

<?php
$xmlfile = 'Books.xml';
$xml = simplexml_load_file($xmlfile);
$namespace = 'http://mynamespace.com';
$xml->registerXPathNamespace('ns',$namespace);
$books = $xml->xpath('//ns:Book');
//loop through each book -
foreach($books as $book) {
$values = $xml->xpath('ns:BookKey/Value');
$bookkeys= array();
//loop through each Book's BookKey Values and push them to array -
foreach($values as $value){  
array_push($bookkeys, $value->attributes()->value);
}    
$sales = $xml->xpath('ns:Sales');
//loop through each Book's Sales and write out Month and Number value after the book key values -
foreach($sales as $sale){  
foreach($bookkeys as $bk){
echo $bk.",";
}
echo $sale->Month;
echo ",";
echo $sale->Number->attributes()->value;
echo "'n";
}
}
?>

从var_dump中，$books数组看起来是ok的;但是，$values和$sales是空数组，因此显然路径是不正确的。我试过离开命名空间，但仍然得到一个空数组;我希望避免取出文件中所有的value和sales实例，而不是只取出每个特定图书的实例。