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

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->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;$i<$words->length;$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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