<a href="?action=up&id=200">上一篇</a>
<a href="?action=down&id=200">下一篇</a>

$id= $_GET['id'];
//上一篇：
$sql= select * from article where id < '.$id.' order by id desc limit 0,1';
$rs= mysql_query($sql);
$row= mysql_fetch_array ($rs);
//下一篇：
$sql= select * from article where id < '.$id.' order by id asc limit 0,1';
$rs= mysql_query($sql);
$row= mysql_fetch_array ($rs);

<?php
 echo GetPreNext(pre,news,$_REQUEST[catid],$_REQUEST[id]);?>
//显示上一篇下一篇
 function GetPreNext($gtype,$table,$catid,$id){
 $preR=mysql_fetch_array(mysql_query("select * from ".$table." where catid=".$catid." and id<$id order by id desc limit 0,1"));//id比传入id小的最近一条
 $nextR=mysql_fetch_array(mysql_query("select * from ".$table." where catid=".$catid." and id>$id order by id asc limit 0,1"));//id比传入id大的最近一条
  $next = (is_array($nextR) ? " where id={$nextR['id']} " : ' where 1>2 ');
  $pre = (is_array($preR) ? " where id={$preR['id']} " : ' where 1>2 ');
   $query = "Select * from ".$table." ";
      $nextRow =mysql_query($query.$next);
      $preRow = mysql_query($query.$pre);
      if($PreNext=mysql_fetch_array($preRow))
      {
       echo $PreNext['pre'] = "上一篇：<a href='newsshow.php?id=".$preR['id']."&&catid=".$catid."'>".$PreNext['title']."</a> ";
      }
      else
      {
       echo $PreNext['pre'] = "上一篇：没有了 ";
      }
      if($PreNext=mysql_fetch_array($nextRow))
      {
       echo $PreNext['next'] = "下一篇：<a href='newsshow.php?id=".$nextR['id']."&&catid=".$catid."'>".$PreNext['title']."</a> ";
      }
      else
      {
        echo $PreNext['next'] = "下一篇：没有了 ";
      }
}