ajax-(Myxmlhttprequest.responsexml return value is null problem

Ajaxphpjshtml

The

Code is:
showcities.php:

  
      ---Province---  Zhejiang   Jiangsu     --County--   

Server code is like this:
showcitypro.php:

 !--? php header (" Content-type:text/xml;charset=utf-8 ");    Header ("Cache-control:no-cache");    Receive user-selected province $province =$_post[' province '];    echo "received the data is:"; ?--> Window.alert (Myxmlhttprequest.responsexml); received is always null  

 Why is this? Novice, so there is no reward. Ask the great God for help. I'm going to be tortured to death by this mistake ... 

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