php-新学的PHP，但还是不会连接数据库，有个小问题

php数据库

问一下怎么建立数据库呢？登陆时已经可以完成的了，就是留言功能不能实现

<code> <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>留言板</title>
<div> <a href="denglu.php">返回</a>
</div>
<?phperror_reporting (E_ALL &~ E_NOTICE);session_start(); $con = mysql_connect("localhost","root","");  if (!$con)    {    die('Could not connect: ' . mysql_error());    }  else echo"<br>connected!<br>";  $selected = mysql_select_db("center" ) ;//if($selected)//echo"selected!<br>";mysql_query("set names 'utf8'");$insert_account = $_SESSION['account'];$insert_pwd = $_SESSION['password'];echo $insert_account;//echo $_POST[comment];if($_SESSION['check'] ==1){if(!empty($_POST[comment]) ){$sql="INSERT INTO comment (account,comment,time)VALUES('$insert_account','$_POST[comment]','$printtime')";  mysql_query($sql) or die(mysql_error());$_SESSION['check'] = 0;}else echo '<div align="center">
<font color="red" font size="50">欢迎来到留言板</font>' ;}$insert_account?><div align="center"><form method="post" action="<?php%20echo%20htmlspecialchars(%24_SERVER%5B" php_self>"><div valign="center">请留言 </div> <textarea name="comment" rows="5" cols="40"><?php //echo readfile("webdictionary.txt");?></textarea><tr><input type="submit" name="submit" value="提交"></tr>
<p>您已提交的留言：</p>
<hr>
<?php $pagesize=10;mysql_select_db("center") or die("no");//取得记录总数$rs，计算总页数用$rs=mysql_query("select count(*) from comment");$myrow = mysql_fetch_array($rs);$numrows=$myrow[0];echo "总留言数量：".$numrows;//计算总页数$pages=intval($numrows/$pagesize);if ($numrows%$pagesize)$pages++;//设置页数if (isset($_GET['page'])){$page=intval($_GET['page']);}else{//设置为第一页 $page=1;}//计算记录偏移量$offset=$pagesize*($page - 1);//读取指定记录数$rs=mysql_query("select * from comment order by ID desc limit $offset,10");//$offset,if ($myrow = mysql_fetch_array($rs)){ $i=0;?><table border="0" width="100%">
<tr>
<td width="50%" bgcolor="#E0E0E0">    <p align="center">留言</p>
</td>
<td width="20%" bgcolor="#E0E0E0">    <p align="center">发布时间</p>
</td>
<td width="20%" bgcolor="#E0E0E0">    <p>留言用户</p>
</td>  </tr>  <?php do {    $i++;    ?>  <tr>   <td width="50%"><?php echo $myrow["comment"] ?></td>   <td width="20%"><?php echo $myrow["time"] ?></td>    <td width="20%"><?php echo $myrow["account"] ?></td>  </tr>   <?php }   while ($myrow = mysql_fetch_array($rs));    echo "</table>";  }  echo "<div align="center">共有".$pages."页(".$page."/".$pages.")";  for ($i=1;$i[".$i ."] ";   echo "[".$page."]";   for ($i=$page+1;$i[".$i ."] ";    echo "</div>";   ?><br><br><br>
</table>
</form></div>
</div></code>