(PHP 5 >= 5.5.0, PHP 7)
json_last_error_msg — Returns the error string of the last json_encode() or json_decode() call
   Returns the error string of the last json_encode() or json_decode()
   call, which did not specify JSON_THROW_ON_ERROR.
  
此函数没有参数。
   Returns the error message on success, "No error" if no
   error has occurred,  或者在失败时返回 FALSE.