Check if Child node exists and get parent node in xml

I have loaded a xml file and I want a specific node when a child node exists.

For example: if the supplierItem->supplier->id is "0101" I want to get the stock "1" out of this specific supplierItem. So I need the parent stock node of the specific supplier id.

My xml looks something like this

<product>
     <supplierItem>
        <id>0001</id>
        <supplier>
           <id>0101</id>
           <name>Company_1</name>
        </supplier>
        <supplierItemId>Product_1_ID</supplierItemId>
        <productName>Product_1</productName>
        <stock>1</stock>
     </supplierItem>
        <id>0002</id>
        <supplier>
           <id>0202</id>
           <name>Company_2</name>
        </supplier>
        <supplierItemId>Product_1_ID</supplierItemId>
        <productName>Product_1</productName>
        <stock>2</stock>
     </supplierItem>
        <id>0003</id>
        <supplier>
           <id>0303</id>
           <name>Company_3</name>
        </supplier>
        <supplierItemId>Product_1_ID</supplierItemId>
        <productName>Product_1</productName>
        <stock>3</stock>
     </supplierItem>
 </product>

My first attemp were:

 $stocks = $xml->xpath("/product/supplierItem");
    foreach ($stocks as $stock)
    {
         echo($stock);   // get all stocks
    }

Any idea? I stuck with it...
Thanks in advance