PHP determines whether str string is xml format data

<?php
//自定义xml验证函数xml_parser()
function xml_parser($str){
    $xml_parser = xml_parser_create();
    if(!xml_parse($xml_parser,$str,true)){
      xml_parser_free($xml_parser);
      return false;
    }else {
      return (json_decode(json_encode(simplexml_load_string($str)),true));
    }
}
//应用示例：
$xmlstr=<<<ETO
<?xml version="1.0" encoding="UTF-8"?>
<books>
  <book>
    <author>Jack Herrington</author>
    <title>PHP Hacks</title>
    <publisher>O&#39;Reilly</publisher>
  </book>
  <book>
    <author>Jack Herrington</author>
    <title>Podcasting Hacks</title>
    <publisher>O&#39;Reilly</publisher>
  </book>
  <book>
    <author>XML数据</author>
    <title>脚本之家</title>
    <publisher>tools.jb51.net</publisher>
  </book>
</books>
ETO;
$jsonstr=&#39;{ "tools": [ { "name":"css format" , "site":"http://tools.jb51.net/code/css" }, { "name":"json format" , "site":"http://tools.jb51.net/code/json" }, { "name":"pwd check" , "site":"http://tools.jb51.net/password/my_password_safe" } ] }&#39;;
if(xml_parser($xmlstr)){
 echo "\$xmlstr是xml格式数据";
}else{
 echo "\$xmlstr不是xml格式数据";
}
echo "<br/>";
if(xml_parser($jsonstr)){
 echo "\$jsonstr是xml格式数据";
}else{
 echo "\$jsonstr不是xml格式数据";
}
?>

$xmlstr是xml格式数据
$jsonstr不是xml格式数据