php针对数据库的读取、判断及session登陆的使用方法

<?php
  $conn=mysql_connect("localhost","root","admin");
  mysql_select_db("songyunb_development",$conn);
  $id=$_GET["id"];
  $sql="insert into email (sender_id,accepter_id,flag) values (&#39;".$_SESSION["id"]."&#39;,&#39;".$id."&#39;,&#39;no&#39;)";
  $query=mysql_query($sql);
  if($query)
  {
    echo "<script>alert(&#39;?????????&#39;);</script>";
    echo "<script>window.location.href=&#39;reg.php&#39;</script>";
  }
?>

<?php
  $conn=mysql_connect("localhost","root","admin");
  mysql_select_db("songyunb_development",$conn);
  $sql="select * from comments";
  $query=mysql_query($sql);
  while($out=mysql_fetch_array($query))
  {
    echo $out["content"]."-------<a href=&#39;newfile.php?id=".$out["id"]."&#39;>查看邮件</a><br/>";
  }
?>

<?php
 $conn=mysql_connect("localhost","root","admin");
  mysql_select_db("songyunb_development",$conn);
 $result="";
 $id="";
 if(isset($_GET["id"]))
 {
  $id=$_GET["id"];
  $sql="select * from comments where id=&#39;".$_GET["id"]."&#39;";
  $result=mysql_query($sql);
 }
$out=mysql_fetch_array($result);
echo $out["content"]."<br/>";
echo $out["created_at"]."<br/>";
echo "<a href=&#39;send_email.php?id=".$out["id"]."&#39;>发送邮件</a><br/><hr>";
//看看有没有新邮件
$sql_search_email="select * from email where accepter_id=&#39;".$id."&#39;";
$query=mysql_query($sql_search_email);
$result_email=mysql_fetch_array($query);
if($result_email["accepter_id"]==$_SESSION["id"]&&$result_email["flag"]=="no")
{
  echo "<strong><a href=&#39;see_email.php?id=".$result_email["id"]."&#39;>您有新邮件</a></strong>";
}
?>

<?php
$conn=mysql_connect("localhost","root","admin");
  mysql_select_db("songyunb_development",$conn);
  $_SESSION["id"]=15;
  echo "<a href=&#39;delete_session.php&#39;>清除session</a>";
  echo "<a href=&#39;reg.php&#39;>重新注册</a>";
?>

<?php
  if(isset($_SESSION["id"]))
  {
   unset($_SESSION["id"]);
  }
  echo "<script>alert(&#39;清除成功&#39;);</script>";
?>