重定向到网页(html)

我在php中见过使用header来重定向到一个页面。

我已经使用了下面代码中的方法和下面代码中给出的a重定向错误。

我需要知道代码出了什么问题。

<?php
/**
 * Created by PhpStorm.
 * User: Priyabrata
 * Date: 3/18/14
 * Time: 8:58 PM
 */

function connect_to_db($uid, $pass){
    $con=mysqli_connect("localhost", "root", "12345", "MyDB");
    if (mysqli_connect_errno()){
        echo "Failed to establish link!!";
    }else{
        echo mysqli_connect_error();
    }
    $result=mysqli_query($con, "SELECT `pwd` from `login_table` where `uid` = "."'".$uid."'");
    if (!$result){
        echo "Error!!";
        exit();
    }
    while($xx = mysqli_fetch_array($result)){
        if ($xx['pwd'] == hash("MD5", $pass)){
            header("location:../Html/Login-Existing-Users1_success.htm");
            exit();
        }else{
            echo "Invalid Login Credentials!!";
            exit();
        }
    }
    mysqli_close($con);
}
function validate_credentials(){
    if ((isset($_POST["username"]))&&(isset($_POST["password"]))){
        $uid = $_POST["username"];
        $pwd = $_POST["password"];
        if (($uid == "")||($pwd == "")){
            echo "Please enter User name and password";
        }else{
            //Check user name and password
            connect_to_db($uid, $pwd);
        }
    }else{
        echo "Please enter User name and password";
    }
}
validate_credentials();

这是我得到的错误，而不是重定向时，上传到服务器:

Warning: Cannot modify header information - headers already sent by (output started at /home/opticfhb/public_html/helpvssupport.net/login.php:19) in /home/opticfhb/public_html/helpvssupport.net/login.php on line 27

附加信息:这在我的本地机器上工作得很好，在服务器上产生了问题。