$xml = simplexml_load_string($xmlstring);
$json = json_encode($xml);
//$array = json_decode($json,TRUE);

案例：
$xml = '<?xml version="1.0" encoding="utf-8"?><root><a/><b>text</b><c name="value" /><d name="value">text</d><e><a>text1</a><b>text2</b></e><f><a>text1</a><a>text2</a></f><g>text1<a>text2</a></g></root>';
$xmldoc = simplexml_load_string($xml);
$json = json_encode($xmldoc);
echo $json;

输出结果：
{"a":{},"b":"text","c":{"@attributes":{"name":"value"}},"d":"text","e":{"a":"text1","b":"text2"},"f":{"a":["text1","text2"]},"g":"text1"}

<?xml version="1.0" encoding="utf-8"?>
<root>
  <a/>
  <b>text</b>
  <c name="value" />
  <d name="value">text</d>
  <e>
    <a>text1</a>
    <b>text2</b>
  </e>
  <f>
    <a>text1</a>
    <a>text2</a>
  </f>
  <g>
    text1
    <a>text2</a>
  </g>
</root>

可见json_encode编码XML还是有bug的。至少d标签的属性丢失了，g标签下的a标签全部丢失了

<?php
$xml = '<?xml version="1.0" encoding="utf-8"?><root><node id="15">E</node></root>';
$xmldoc = simplexml_load_string($xml);
$json = json_encode($xmldoc);
echo $json;
?>
输出结果：{"node":"E"}
可见id属性被丢掉了

http://php.net/manual/en/function.json-encode.php
4 Ray.Paseur often uses Gmail ¶7 months ago If you're wondering whether a JSON string can be an analog of an XML document, the answer is probably "nope."  XML supports attributes, but JSON does not.  A JSON string generated by json_encode(), when called on a SimpleXML object, will not have the attributes and no error or exception will issue - the original data will simply be lost.  To see this in action:
<?php
error_reporting(E_ALL);
echo '<pre>';

// STARTING FROM XML
$xml = <<<EOD
<?xml version="1.0" ?>
<ingredients>
  <ingredient>
     <name>tomatoes</name>
     <quantity type="cup">4</quantity>
  </ingredient>
  <ingredient>
     <name>salt</name>
     <quantity type="tablespoon">2</quantity>
  </ingredient>
</ingredients>
EOD;

// CREATES AN ARRAY OF SimpleXMLElement OBJECTS
$obj = SimpleXML_Load_String($xml);
var_dump($obj);
echo PHP_EOL;

// SHOW THE ATTRIBUTES HIDDEN IN THE SimpleXMLElement OBJECTS
foreach ($obj as $sub)
{
    echo PHP_EOL . (string)$sub->quantity . ' ' . (string)$sub->quantity['type'];
}
echo PHP_EOL;

// USING THE OBJECT, CREATE A JSON STRING
$jso = json_encode($obj);
echo htmlentities($jso); // 'type' IS LOST
echo PHP_EOL;


PHP官网上的一个user contributed note 也提到了type属性丢失

所以，如果用了这种方法转换，那么在XML中应该避免使用以下格式：
<e name="value">text</e>
<e> text <a>text</a> </e>

http://blog.longwin.com.tw/2006/10/php_xml2json_2006/
这里还有一篇文章