读取XML获取不了节点值?

<?php
$reader = new XMLReader();
$reader->open("1.xml");                                                     
while($reader->read()){
if($reader->nodeType==XMLREADER::ELEMENT) {
             if ($reader->localName == "humans") {
                 //已经能进if语句，但是获取不了value
                 echo $reader->value;
             }
    }
}
?>

XML文件1.xml：

<?xml version="1.0"?>
<!DOCTYPE PubmedArticleSet PUBLIC "-//NLM//DTD PubMedArticle, 1st January 2015//EN" "http://www.ncbi.nlm.nih.gov/corehtml/query/DTD/pubmed_150101.dtd">
  <humans>
  hello
  </humans>

$reader->value无效，什么原因？请教！

---

if语句少了$reader->read()；