为什么同一个字符串两次MD5加密出来的值不同?

今天在写自己的网站时遇到了个让我很困惑的问题. 情况是这样的. 我想通过发送邮件验证用户的邮箱, 第一次通过MD5生成URL发送了邮件

 $_ZhuCeShiJian = time(); $_YanZheng = md5($_POST['zhanghao'] . $_YanZhengMa . $_ZhuCeShiJian); $_ChaXun_LeiXing = 'INSERT INTO ' . QIANZHUI . '_YongHuLeiXing (YongHu_ID, LeiXing_ID, JiHuoMa, YouXiaoQi, ZhuangTai, ZhuCeShiJian) VALUES (?, ?, ?, ?, ?, ?)'; $_ShengMing_LeiXing = $_ShuJuKuLianJie -> prepare($_ChaXun_LeiXing); $_ShengMing_LeiXing -> bind_param('ddsddd', $_ZhangHaoID, $_LeiXingID, $_YanZhengMa, $_YouXiaoQi, $_ZhuangTai, $_ZhuCeShiJian); $_ShengMing_LeiXing -> execute(); $_ShengMing_LeiXing -> close();

$_YanZheng = md5($_ZhangHao . $_YanZhengMa . $_ZhuCeShiJian);