php - 支付宝回调如何验证签名？

支付宝回调，得到这样一个数组（数组只是形式，并不是真实数据）

array (
  'discount' => '0.00',
  'payment_type' => '1',
  'subject' => '测试02',
  'trade_no' => '201603122100106821',
  'buyer_email' => '18776152065',
  'gmt_create' => '2016-03-12 11:30:08',
  'notify_type' => 'trade_status_sync',
  'quantity' => '1',
  'out_trade_no' => '1603125610283',
  'seller_id' => '2088122451677261',
  'notify_time' => '2016-03-13 11:54:40',
  'body' => '测试02',
  'trade_status' => 'TRADE_SUCCESS',
  'is_total_fee_adjust' => 'N',
  'total_fee' => '0.01',
  'gmt_payment' => '2016-03-12 11:30:09',
  'seller_email' => 'xxxx@126.com',
  'price' => '0.01',
  'buyer_id' => '2088612804',
  'notify_id' => 'ba20b13f6lk2',
  'use_coupon' => 'N',
  'sign_type' => 'RSA',
  'sign' => 'Bn6IEyE9=',
)

然后ksort()排序,去除sign_type,sign,拼接成
body=Hello&buyer_email=13788888888&buyer_id=2088002007013600..............这样字符串

openssl_verify(`拼接字符串`, base64_decode($_POST['sign']), $publickey);

总返回 0 ，验证不成功，是哪一步出错