PHP SESSION 变量使用 $_SESSION

我正在尝试使用 $_SESSION 传递会话变量，因此我可以检查当前登录的用户是否具有"管理员"权限，该权限存储在我的数据库表"login"中作为"login_privileges"。如果他们具有管理员权限，则可以访问某些页面，如果没有，则只是拒绝访问，但不会注销。在表单中输入有效的用户名和密码时，我收到"无法在写入上下文中使用函数返回值"错误。一定是会话出错了，出了什么问题？

<?php
require("dbconnectprojdev.php");
$con = mysql_connect($host, $username, $password);
$username = $_POST['username']; 
$password = $_POST['password']; 
$username = stripslashes($username); 
$password = stripslashes($password); 
$username = mysql_real_escape_string($username, $con); 
$password = mysql_real_escape_string($password, $con); 
$stmt=$dbh->prepare("SELECT * FROM login WHERE login_username = :username AND login_password = :password");
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
$stmt->execute();
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
$count = $count + 1;
}
$privilege = $row['login_privileges'];
if($count==1){  
session_start();
$_SESSION('privilege') = $privilege;
$_SESSION('username') = $username; 
$_SESSION('password') = $password; 
header("location:logincorrect.php"); 
} else { 
echo "Invalid login details"; 
echo "<b></br><a href=default.php>Back to login</a>";
} 
?>  

"dbconnectprojdev.php"看起来像：

<?php
$host = "*****";
$database = "*****";
$username = "*****";
$password = "*****";
$dbh = new PDO("mysql:host=$host; dbname=$database; charset=utf8", $username , $password );
?>