in Wechat"s payment callback, after I successfully processed it, I directly 
 return "success" 
 Wechat reported an error 
 error example: 
 transaction_id=4200000217201810305986075380] > < / return_code > < return_msg >  < / return_msg > < / xml >] 
I change it to xml format and return
return sprintf ("< xml > < return_code > < / return_code > < return_msg > < / return_msg > < / xml >");
Wechat still reported an error
 transaction_id=4200000221201810316667479789] > < / return_code > < return_msg >  < return_code >  < / return_code > < return_msg >  < / return_msg > < / xml >]] > < / return_msg > < / xml >] 
 how to solve 
