easy,  code below: 

<?
require_once("./config/config.php");
ob_start();
$id=$_GET[id];
$sql="select * from test where id='$id'";
$result=mysql_query($sql);
$rs=mysql_fetch_object($result);
$smarty->assign("showtitle",$rs->title);
$smarty->assign("showcontent",$rs->content);
$smarty->display("content.html");
$this_my_f= ob_get_contents();
ob_end_clean();
$filename = "$id.html";
tohtmlfile_cjjer($filename,$this_my_f);
//文件生成函数
function tohtmlfile_cjjer($file_cjjer_name,$file_cjjer_content)
{
if (is_file ($file_cjjer_name)){
      @unlink ($file_cjjer_name); //存在，就删除
}
$cjjer_handle = fopen ($file_cjjer_name,"w"); //创建文件
if (!is_writable ($file_cjjer_name)){ //判断写权限
      return false;
}
if (!fwrite ($cjjer_handle,$file_cjjer_content)){
      return false;
}
fclose ($cjjer_handle); //关闭指针
return $file_cjjer_name; //返回文件名
}
?>