如何在输出后设置重定向？

Question

已登录，如果你设置了POST，你将重定向到一个页面......但标题（＆＃34; Location：page.php＆＃34;）;似乎没有用，我也不知道为什么......你能帮帮我吗？

以下代码：

$username = mysql_real_escape_string($_POST['username']);
  $password = mysql_real_escape_string($_POST['password']);
  $passwordantiscam = mysql_real_escape_string($_POST['passwordantiscam']);

  $usernamep = mysql_query("SELECT username FROM users WHERE username = '".$username."' AND rank > 7");
  $passwordp = mysql_query("SELECT password FROM users WHERE username = '".$username."' AND password = '".$password."' AND rank > 7");
  $passwordantiscamp = mysql_query("SELECT password_antiscam FROM admin_pswantiscam WHERE password_antiscam = '".$passwordantiscam."'");

  if(isset($_POST['login'])) {

      if(empty($username) && empty($password) && empty($passwordantiscam)) {
          echo '<div class="error">Hai lasciato qualche campo vuoto</div>';
      } else {

      if(mysql_num_rows($usernamep) == 0) {
          echo '<div class="error">Nome utente sbagliato</div>';
      } else {
          if(mysql_num_rows($passwordp) == 0) {
          echo '<div class="error">Password sbagliata</div>';
          } else {
              if(mysql_num_rows($passwordantiscamp) == 0) {
                  echo '<div class="error">Password anti-scam sbagliata</div>';
              } else {
                  $check = mysql_query("SELECT id,username FROM users WHERE username = '" . $username. "' AND password = '" . $password. "' AND rank > 7 LIMIT 1") or die(mysql_error());
                  $row = mysql_fetch_assoc($check);
                  session_start();
                  $_SESSION['acp'] = true;
                  $_SESSION['hkusername'] = $row['username'];
                  $_SESSION['hkpassword'] = $password;
                  $my_id = $row['id'];
                  if(!session_is_registered(username)){  
                    $_SESSION['username'] = $row['username'];
                    $_SESSION['password'] = $password;
                  }
                  header("Location: home.php");
                  mysql_query("INSERT INTO system_stafflog (action,message,note,userid,targetid,timestamp) VALUES ('Housekeeping','".$_SESSION['hkusername']." authenticated from ".$remote_ip."','login.php','".$my_id."','','".$date_full."')") or die(mysql_error());
                  exit;
            }
          }
      }
      }
 }

我已经读过标题必须放在输出之前..但如果我这样做，代码将重定向BEFORE以启动会话...我该如何解决这个问题？