用PHP解析XML失误，尝试用XML做个数据源

用PHP解析XML出错，尝试用XML做个数据源
利于XML词库，用PHP处理时出现
Notice: Trying to get property of non-object in C:\apache\htdocs\olword\wordprocess.php Fatal error: Call to a member function getElementsByTagName() on a non-object in C:\apache\htdocs\olword\wordprocess.php 

PHP code

<!--

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-->if($type=="update"){
     //接收新的单词

  $update_word_en=$_REQUEST['enword'];
  $update_word_ch=$_REQUEST['chword'];
 
  //查询
  $words=$xmldoc->getElementsByTagName("word");
  $isEnd=false;
  //遍历
 
  
  for($i=0;$ilength;$i++){
  //依次取出单词
  $word=$words->item($i);
  $word_en=getNodeVal($word,"en");
  if($update_word_en==$word_en){
      $isEnd=true;
   / /这里出现错误
      $parent_word=$word_en->parentNode;
      $word_ch=$parent_word->getElementsByTagName("ch")->item(0);
      $word_ch->nodeValue=$update_word_ch;
      //$word_en->nextSibling->nodeValue=$update_word_ch;这样修改也有问题
      //更新文件
   $isUpateOk=$xmldoc->save("word.xml");
if( $isUpateOk){

    echo "更新成功";
      }else echo "更新失败";
   }
}
if(!$isEnd){  
      echo "查无此词";

     }

}

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