[PHP7] Problem (?) med SessionHandlerInterface och $_SESSION

<?php

class Session implements SessionHandlerInterface
{
    protected $pdo;
    
    public function __construct()
    {
        $dsn = 'mysql:host=' . 'localhost' . ';dbname=' . 'bytewizards.se';
        
        $options = array(
            PDO::ATTR_PERSISTENT    => true,
            PDO::ATTR_ERRMODE       => PDO::ERRMODE_EXCEPTION
        );
        
        try {
            $this->pdo = new PDO($dsn, '********', '********', $options);
        }
        
        catch(PDOException $exception) {
            echo $exception->getMessage();
        }
    }
    
    public function open($save_path, $session_name)
    {
        if($this->pdo) {
            return true;
        } else {
            return false;
        }
    }
    
    public function close()
    {
        $this->pdo = null;
        return true;
    }
    
    public function read($session_id)
    {
        $statement = $this->pdo->prepare('SELECT data FROM sessions WHERE id = :session_id');
        $statement->bindParam(':session_id', $session_id);
        $statement->execute();

        return $statement->fetchColumn(); 
    }
    
    public function write($session_id, $session_data)
    {
        $session_timestamp = time();
        
        $statement = $this->pdo->prepare('REPLACE INTO sessions VALUES(:session_id, :session_timestamp, :session_data)');
        $statement->bindParam(':session_id', $session_id);
        $statement->bindParam(':session_timestamp', $session_timestamp);
        $statement->bindParam(':session_data', $session_data);
        
        return $statement->execute();
    }
    
    public function destroy($session_id)
    {   
        $statement = $this->pdo->prepare('DELETE * FROM sessions WHERE id = :session_id');
        $statement->bindParam(':session_id', $session_id);
        
        return $statement->execute();
    }
    
    public function gc($maxlifetime)
    {
        $session_timestamp_timeout = time() - $maxlifetime;
        
        $statement = $this->pdo->prepare('DELETE * FROM sessions WHERE timestamp < :session_timestamp_timeout)');
        $statement->bindParam(':session_timestamp', $session_timestamp_timeout);
        
        return $statement->execute();
    }
 }

[file1.php]
<?php
   $session =  new Session();
   session_set_save_handler($session, true);
   session_start();
   
   $_SESSION["SESSION_DATA"] = 663154;   

[file2.php]
<?php
   $session =  new Session();
   session_set_save_handler($session, true);
   session_start();
   
   echo $_SESSION["SESSION_DATA"]; // 663154