ใช้คำสั่ง MySql ลบชื่อผู้ใช้จากฐานข้อมูล แล้ว session ของเราหายไปด้วย แก้อย่างไรครับ

<?php
session_start();
	if($_SESSION['username'] == "")
	{
		echo "<script>alert('Please Login!'); window.location='SkinRep-Login.php'</script>";
		exit();
	}
	if($_SESSION['status'] != "admin")
	{
		echo "<script>alert('This page is reserved for admin ^-^')</script>";
	}
	
$username = $_POST['username'];

include('db_connector.php');
$checkuser="select*from userinfo where username='$username'";
$query=mysql_db_query($dbname,$checkuser);
$usernum=mysql_num_rows($query);

	if($usernum == 1){
		$sql="delete from userinfo where username='$username'";
		mysql_query("SET NAMES UTF8");
		$sqlquery=mysql_db_query($dbname,$sql);
		$checkdel="select*from userinfo where username='$username'";
		$result=mysql_db_query($dbname,$checkdel);
		$num=mysql_num_rows($result);
		
			if($num == 0){
				echo "<script>alert('The user has been deleted'); window.location='removeuser.php'</script>";
			}
			else{
				"<script>alert('Unable to delete the user'); window.location='removeuser.php'</script>";
			}
	}
	elseif($usernum == 0){
		echo "<script>alert('No user in the database'); window.location='removeuser.php'</script>";
	}
	else{
		echo "<script>alert('Error!'); window.location='removeuser.php'</script>";
	}
mysql_close();
?>