微信退款机制

/**
 * 处理退款
 * @param $out_trade_no
 * @param $total_fee
 * @param $refund_fee
 * @param $from 1余额 2未结算
 * @return array
 * @throws Exception
 * 策略一：当天支付的钱，从未结算中退；非当天支付的钱，从余额中退（结算的钱到余额中有个缓冲期1-3天，结算到余额要收千分之一的手续费）。确保退款正常，需要在余额中留有备用金。
 * 策略二：优先从未结算中退，未结算中余额不足，再从余额中退。（需要查询两次，比较消耗网络。好处就是可以节省被腾讯收取的千分之一的费用。）
 */
public static function refundOrder($out_trade_no,$total_fee,$refund_fee,$from = 1) {
    $refund_no = $out_trade_no.$total_fee;
    if ((int)$from === 1) {
        $refund_account = 'REFUND_SOURCE_RECHARGE_FUNDS';
    } else {
        $refund_account = 'REFUND_SOURCE_UNSETTLED_FUNDS';
    }
    // $refund_account = 'REFUND_SOURCE_UNSETTLED_FUNDS';
    $param = array(
        'appid'         => C('APPID'),
        'mch_id'        => C('MCHID'),
        'nonce_str'     => self::createNonceStr(),
        'out_refund_no' => $refund_no, //由后端生成的退款单号，需要保证唯一，因为多个同样的退款单号只会退款一次。
        'out_trade_no'  => $out_trade_no,                   //退款订单在支付时生成的订单号
        'total_fee'     => $total_fee,
        'refund_fee'    => $refund_fee,
        'refund_account'=> $refund_account,  // REFUND_SOURCE_RECHARGE_FUNDS 从余额退，REFUND_SOURCE_UNSETTLED_FUNDS 从未结算退
        'op_user_id'    => C('MCHID'),          //操作员 op_user_id .与商户号相同即可
    );

    $param['sign'] = self::MakeSign($param);
    $xml_data = self::ToXml($param);
    $xml_result = self::postXmlSSLCurl($xml_data,'https://api.mch.weixin.qq.com/secapi/pay/refund');
    $result = self::FromXml($xml_result);
    self::setlog($param,$result,__METHOD__);
    if (!$result){
        $result_arr = [
            'num'     =>      '0',
            'desc'   =>      '接口错误',
        ];
        return $result_arr;
    }

    if ($result['result_code'] != 'SUCCESS'){
        $result_arr = [
            'num'     =>      '-1',
            'desc'    =>      $result['err_code_des'],
            'err_code' =>     $result['err_code'], // NOTENOUGH 余额不足
        ];
    } else {
        $result_arr = [
            'num'     =>      '1',
            'desc'    =>      '退款成功',
            'refund_id'    =>      $result['refund_id'],
            'refund_no'    =>      $refund_no,
        ];
    }

    return $result_arr;
}

经验之谈！