PHP XML DOM未捕获异常'文档错误'

我正在尝试学习XML，我知道这是一个没有正确导入节点的问题。但我不太明白。我一直在寻找，大多数人没有像我这样有多个子元素的部门。

下面是我的XML结构:

<SOT>  
    <DEPARTMENT name="Aviation Technology" id="AT">  
        <EMPLOYEE type="Faculty">  
            <LOGIN>jdoe1</LOGIN>  
            <NAME>John Doe</NAME>   
        </EMPLOYEE>
        <EMPLOYEE type="Faculty">  
            <LOGIN>jdoe2</LOGIN>  
            <NAME>Jane Doe</NAME>   
        </EMPLOYEE>
        <EMPLOYEE type="Faculty">  
            <LOGIN>jdoe3</LOGIN>  
            <NAME>Joe Doe</NAME>  
        </EMPLOYEE> 
    </DEPARTMENT>    
    <DEPARTMENT name="Building and Construction Management" id="BCM">  
    </DEPARTMENT>
    <DEPARTMENT name="Computer Graphics Technology" id="CGT">  
    </DEPARTMENT>  
</SOT>

我明白SOT是我的根元素，部门是SOT的"子"，每个部门有多个员工的"子"。我遇到的问题是当我试图向某个部门添加新员工时。当我尝试$departmentArray->item($i)->appendChild($employee);时，我得到了错误的文档错误。

我使用这段PHP代码尝试将子节点附加到departmentNode

<?php
    //grab form data
    $username = $_POST['username'];
    $employeeName = $_POST['employeeName'];
    $department = $_POST['department'];
    //create new DOMDocument to hold current XML data
    $doc = new DOMDocument();
    $doc->load("test.xml");
    $xpath = new DOMXpath($doc);
    //create our new DOMDocument for combining the XML data
    $newDoc = new DOMDocument();
    $newDoc->preserveWhiteSpace = false;
    //create School of Tech Node and append to new doc
    $sotElement = $newDoc->createElement("SOT");
    $newDoc->appendChild($sotElement);
    $root = $newDoc->documentElement;
    //grab the department Nodes
    $departmentArray = $doc->getElementsByTagName("DEPARTMENT");
    //create a new employee and set attribute to faculty
    $employee = $newDoc->createElement("EMPLOYEE");
    $employee->setAttribute("type", "Faculty");
    //counters (might use them later for ->item(counter) function
    $indexCounter = 0;
    $i = 0;
    foreach($departmentArray as $departmentNode){
        if(strcmp($departmentNode->getAttribute('name'),$department) == 0){//check if departments match
            //create login element
            $loginNode = $newDoc->createElement("LOGIN");
            $loginNode->appendChild($newDoc->createTextNode($username));
            $employee->appendChild($loginNode);
            //create name node
            $nameNode = $newDoc->createElement("NAME");
            $nameNode->appendChild($newDoc->createTextNode($employeeName));
            $employee->appendChild($nameNode);
            //append employee onto department node
            //$departmentArray->item($i) = $doc->importNode($departmentArray->item($i), true);
            $departmentArray->item($i)->appendChild($employee);
            //set index of department array (possibly used for appending later)
            $indexCounter = $i;
        }
        $i++;
    }
    #######################################
    /*Write out data to XML file         */
    #######################################
    //$departmentArray = $doc->getElementsByTagName("DEPARTMENT");
    foreach($departmentArray as $departmentNode){
        $tempNode = $newDoc->importNode($departmentNode, true);
        /*if(strcmp($departmentNode->getAttribute('name'),$department) == 0){
            $sotElement->appendChild($employee);
        }*/
        $sotElement->appendChild($tempNode);
    }
    $newDoc->formatOutput = true;
    $newDoc->save("test2.xml");

?>

任何帮助解释如何正确导入所有部门节点，以便能够追加到他们将非常感谢。