app_id=12345&out_trade_no=123123123123&amount_total=1
app_id=12345&out_trade_no=123123123123&amount_total=1&key=xxxxxxxxx
$sign = md5('app_id=12345&out_trade_no=123123123123&amount_total=1&key=xxxxxxxxx');
本文档参考 payjs 文档