php无法向mysql中插入值解决方案

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conn.php
<?php
/*
 * Created on 2012-5-7
 *
 * To change the template for this generated file go to
 * Window - Preferences - PHPeclipse - PHP - Code Templates
 */
 $conn=@ mysql_connect("localhost","root","admin") or die("数据库连接失败");
 mysql_select_db("bbs",$conn);
 mysql_query("set names 'GBK'"); //使用GBK中文编码;
?>

add.php
<?php
/*
 * Created on 2012-5-7
 *
 * To change the template for this generated file go to
 * Window - Preferences - PHPeclipse - PHP - Code Templates
 *
 */
 include("conn.php");

 if($_POST['submit']){
  $sql="insert into message (id,user,title,content,lastdate)" .
             "values ('','$_POST[user]','$_POST[title]','$_POST[content]',now())";
  mysql_query($sql);
  echo "发表成功";
 }
?>

  <form action="add.php" method="post" >
   用户：<input type="text" name="user"/><br>
   标题：<input type="text" name="title"/><br>
   内容：<textarea name="content"></textarea><br>
   <input type="submit" name="submit" value="发布留言"/>

  </form>