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Warning: Cannot send session cache limiter - headers already sent (output started at /home/metalnation/public_html/login.htm:6) in /home/metalnation/public_html/login.php on line 52

이런 에러가 나오는군요...
뭐가 잘못되어서 그런거죠?
가르쳐 주세요!

<?php
include "common_db.inc";
$register_script = "log.php";

function auth_user($userid, $userpassword)
  {
  global $default_dbname, $user_tablename;

  $link_id = db_connect($default_dbname);
  $query = "SELECT username FROM $user_tablename WHERE userid='$userid' AND userpassword=password('$userpassword')";
  $result=mysql_query($query);
  if(!mysql_num_rows($result)) return 0;
  else
   {
   $query_data=mysql_fetch_row($result);
   return $query_data[0];
   }
  }
  function login_form()
   {
   global $PHP_SELF;
?>
<form method="POST" action="<? echo $PHP_SELF ?>">
<table width=100% border=0 cellspacing=0 cellpadding=0>
  <tr>
   <td align=left valign=top background="gif/right_bg_bg.gif">
    <table border=0 cellspacing=0 cellpadding=0>
     <tr>
      <td><img src="gif/lo_id.gif"></td>
      <td><input type="text" name="userid" size=8 style='width:80px; height:18px;'></td>
     </tr>
     <tr>
      <td><img src="gif/lo_pass.gif"></td>
      <td><input type="PASSWORD" name="userpassword" size=8 style='width:80px; height:18px;'></td>
     </tr>
     <tr>
      <td align=center vliagn=middle colspan=2>
       <input type=image src="gif/lo_login.gif" border=0 value="LOGIN" name="Submit"></td>
     </tr>
     <tr>
      <td align=center vliagn=middle colspan=2><a href="join.htm"><img src="gif/lo_join.gif" border=0></a></td>
     </tr>
    </table>
   </td>
   <td background="gif/right_bg_bg.gif" align=right width=100%><img src="gif/right_bg.gif"></td>
  </tr>
</table>
</form>
<?
}

session_start();   <---------52라입니다.
if(!isset($userid))
  {
   login_form();
   exit;
  }
  else
  {
   session_register("userid","userpassword");
   $username=auth_user($userid, $userpassword);
   if(!$username)
    {
    session_unregister("userid");
    session_unregister("userpassword");
    echo "인증실패." . "올바른 회원ID와 비밀번호가 아닙니다." . "다음 링크를 클릭해 다시 로그인 하세요.<br>n";
    echo "<a href="$PHP_SELF">로그인</a><br>";
    echo "회원이 아니시라면" . "다음 링크를 클릭해 회원가입을 해주세요.<BR>n";
    echo "<a href="$register_script">회원등록</a>";
    exit;
   }
   else echo "$username 님 어서오세요!";
  }
?>