XML / PhP如何更新节点NextID

Below is my xml table, i have already created php to add a book but i am struggling on how to update the nextID by one let say i add a book at id 101 but how do i update so NextID displays 102 not 101.

<?xml version="1.0"?>
<catalogue>
  <nextID>101</nextID>
  <tasks>
    <task id="100">
      <title>Task Three</title>
      <taskleader>Cara Forbes</taskleader>
      <participants>Alex Lord</participants>
      <targetdate>25/10/2016</targetdate>
      <summary>Test Text Area</summary>
      <status>completed</status>
      <url>www.taskthree.com</url>
    </task>
  </tasks>
</catalogue>

In response to above.

what i have so far whicj correctly adds a task

   $root= $xml->documentElement;
   $nextIDNode=$root->childNodes->item(0);
   $tasks= $root->childNodes->item(1);

    $firstTask=$tasks->childNodes->item(0);
    $newID=(int)$root->childNodes->item(0)->nodeValue;
    $updateID = $newID + 1;
    $newTitle=$_POST["addTitle"];
    $newTLead=$_POST["addLead"];
    $newPart=$_POST["addPart"];
    $newDate=$_POST["addDate"];
    $newSum=$_POST["addSum"];
    $newStat= "new";
    $newUrl=$_POST["addUrl"];


    $titleNode=$xml->createElement("title");
    $titleTextNode=$xml->createTextNode("$newTitle");
    $titleNode->appendChild($titleTextNode);

    $taskLeaderNode=$xml->createElement("taskleader");
    $taskLeaderTextNode=$xml->createTextNode("$newTLead");
    $taskLeaderNode->appendChild($taskLeaderTextNode);

    $taskPartNode=$xml->createElement("participants");
    $taskPartTextNode=$xml->createTextNode("$newPart");
    $taskPartNode->appendChild($taskPartTextNode);
    $taskDateNode=$xml->createElement("targetdate");
    $taskDateTextNode=$xml->createTextNode("$newDate");
    $taskDateNode->appendChild($taskDateTextNode);
    $taskSumNode=$xml->createElement("summary");
    $taskSumTextNode=$xml->createTextNode("$newSum");
    $taskSumNode->appendChild($taskSumTextNode);    
    $taskStatNode=$xml->createElement("status");
    $taskStatTextNode=$xml->createTextNode("$newStat");
    $taskStatNode->appendChild($taskStatTextNode);
    $taskUrlNode=$xml->createElement("url");
    $taskUrlTextNode=$xml->createTextNode("$newUrl");
    $taskUrlNode->appendChild($taskUrlTextNode);


    $newTaskNode=$xml->createElement("task");
    $newTaskNode->setAttribute("id",$newID);
    $newTaskNode->appendChild($titleNode);
    $newTaskNode->appendChild($taskLeaderNode);
    $newTaskNode->appendChild($taskPartNode);
    $newTaskNode->appendChild($taskDateNode);
    $newTaskNode->appendChild($taskSumNode);
    $newTaskNode->appendChild($taskStatNode);
    $newTaskNode->appendChild($taskUrlNode);
    $tasks->insertBefore($newTaskNode,$firstTask);


    $xml->save("tasks.xml");


   $add = "Task added";
   echo $add;
   $xml->save("tasks.xml");