警告:simplexml_load_string() [function.simplexml-load-string]:

我正面临一些问题的代码，请帮助我。我将非常感谢你……

$category = '
<?xml version='1.0' ?>
<document> 
 <Astronomy>
   <items>milky way, comet</items> 
 </Astronomy> 
 <Business>
   <items>abilities, accelerated benefits</items> 
 </Business> 
<Corrections Technology>
   <items>accomplice, addiction, agent</items> 
</Corrections Technology> 
</document>';

$category = simplexml_load_string($category);