header("Content-type:text/html charset=utf8");$mysqli=new mysqli('localhost','root','***','test');if($mysqli->connect_errno){ ? ?die("Connect Error:".$mysqli->connect_error);}$mysqli->set_charset('utf8');$username=$_POST['username'];$password=md5($_POST['password']);$sql="SELECT * FROM t1 WHERE username=? AND password=?";$mysqli_stmt=$mysqli->prepare($sql);$mysqli_stmt->bind_param('ss',$username,$password);if($mysqli_stmt->execute()) { ? ?$mysqli_stmt->store_result(); ? ?if($mysqli_stmt->num_rows>0){ ? ? ? ?echo "登陸成功"; ? ?}else{ ? ? ? ?echo "登陸失敗"; ? ?}}如以上代碼，為什么給password加上md5數據庫就獲取密碼失敗并且登錄失敗，而把md5去掉并且不使用md5加密passwrod就可以登陸成功，這是為什么？