Get information from another table

Good people, I have the following code:

<?php
require('../members/inc/config.php');
require('../lib/framework.php');

$author = $_SESSION['username'];

if(!empty($_POST['news_title']) || !empty($_POST['news_message']))
{
    if(!empty($_POST['news_title']) && !empty($_POST['news_message']))
    {

            $date = date( 'jS F Y' );
            $time = date( 'H:i' );
            $query = "INSERT INTO news (id, author, title, text, date, time) VALUES('', '".$author."', '".$_POST['news_title']."', '".$_POST['news_message']."', '".$date."', '".$time."')" or die(mysql_error());
            $insert = mysql_query($query) or die(mysql_error());
            echo '<p>Successful News Update&nbsp;&nbsp;&ldquo;'.$_POST['news_title'].'&rdquo;';

    }
    else
    {
        echo '<p>Please fill in all fields</p>';
    }
}
?>

The question is: I have two tables in the same database, q "news" and "members". I want to insert this in the session already started in "members" But it says that it does not know the row "username" and it exists ..

  

Notice: Undefined index: username

Edit: Already, could you help me pass this on to PDO? : ss