PHP Query XML for a Node Attribute value and delete the Node

<issues>  
<issue acc_num="5950001">
    <username>user1</username>
    <password>user1</password>
    <acc_num>5950001</acc_num>
    <pat_num>S009000</pat_num>
    <study_date>February 18, 2009</study_date>
    <sup_name>super</sup_name>
    <sup_nuid>00004</sup_nuid>
    <modality_name>Bmc</modality_name>
    <severity>3</severity>
    <issue_title>QC-MARKER</issue_title>
    <issue_desc>
TESTing layout 2 </issue_desc>
  </issue>
  <issue acc_num="5950002">
    <username>user1</username>
    <password>user1</password>
    <acc_num>5950002</acc_num>
    <pat_num>S001000</pat_num>
    <study_date>February 18, 2009</study_date>
    <sup_name>super</sup_name>
    <sup_nuid>00005</sup_nuid>
    <modality_name>MRc</modality_name>
    <severity>3</severity>
    <issue_title>QC-ARTIFACT</issue_title>
    <issue_desc>TESTing layout 3</issue_desc>
  </issue>
</issues>
 
**************  Current Code  ***********************
        $xmlfile =  'external/external_post.xml';  
                
        $xmlDoc = new DomDocument; 
        $xmlDoc->validateOnParse = true; 
        $xmlDoc->Load($xmlfile); 
        $xmlDoc->preserveWhiteSpace = FALSE; 
        $xpath = new DOMXpath($xmlDoc);
 
        
         
             foreach($validData as $data)  {
              //Keep the bad data in the file untill it can be fixed   
                  $gooddata_holder = $data;
                  $nodelist = $xpath->query('//issue[@acc_num="' . $data . '"]');
                                                                              
                    if ($nodeList->length) {
                        $node = $nodeList->item(0);
                        $node->parentNode->removeChild($node);
                    }
              }
 
$dom->save($xmlfile) or die ("Error saving to file");