header()误用

<?php
header('content-type:text/html;charset=utf-8');
include 'connection.php';
//header('content-type:text/html;charset=utf-8');
$username = isset($_POST['username'])?$_POST['username']:"";
$password = isset($_POST['password'])?$_POST['password']:"";
$username = trim($username);
$password = trim($password);
if($username==""||$password=="")
{
    echo "<script>alert('用户名或密码为空，请重新输入！');history.back();</script>";

}
else{
    $password = md5($password);

    $query = "select * from user where name_user=$username;";
    $result = mysqli_query($link, $query);
    if($result&&mysqli_num_rows($result))
    {
        $data = mysqli_fetch_assoc($result);
        if($password==$data['password_user'])
        {
            setcookie("username",$username,time()+60*60*24);
            setcookie("password",$password,time()+60*60*10);
            mysqli_close($link);
            header("location:view.php");
        }
        else
        {
            //echo "<script>alert('密码错误，请重新输入密码！');history.back();</script>";
             echo "<script>alert('用户名不存在，请去注册！');</script>";
             header("location:sign.php");/////////这里出了一点问题，执行了跳转，但是前面的警告框没有效果
        }
    }
    else
    {
        echo "<script>alert('用户名不存在，请去注册！');location.href='sign.php';</script>";
    }
}
?>