php通过正则表达式记取数据来读取xml的方法

<?xml version="1.0 encoding="utf-8"?>
<humans>
<zhangying>
<name>张映</name>
<sex>男</sex>
<old>28</old>
</zhangying>
<tank>
<name>tank</name>
<sex>男</sex>
<old>28</old>
</tank>
</humans>

<?php
 $xml = "";
 $f = fopen('person.xml', 'r');
 while($data = fread($f,4096)){
  $xml .= $data;
 }
 fclose( $f );
// 上面读取数据
 preg_match_all("/\<humans\>(.*?)\<\/humans\>/s",$xml,$humans); 
//匹配最外层标签里面的内容
 foreach( $humans[1] as $k=>$human )
 {
  preg_match_all("/\<name\>(.*?)\<\/name\>/",$human,$name);
//匹配出名字
  preg_match_all("/\<sex\>(.*?)\<\/sex\>/",$human,$sex);
//匹配出性别
  preg_match_all("/\<old\>(.*?)\<\/old\>/",$human,$old);
//匹配出年龄
 }
foreach($name[1] as $key=>$val){
 echo $val." - ".$sex[$key][1]." - ".$old[$key][1]."<br>" ;
}
?>