Calling all PHP experts! PDO to mysql_connect conversion

<?php 

    // These variables define the connection information for your MySQL database 
    $username = "root"; 
    $password = "root"; 
    $host = "localhost"; 
    $dbname = "users_db"; 

    // UTF-8 is a character encoding scheme that allows you to conveniently store 
    // a wide varienty of special characters, like ¢ or €, in your database. 
    // By passing the following $options array to the database connection code we 
    // are telling the MySQL server that we want to communicate with it using UTF-8 
    // See Wikipedia for more information on UTF-8: 
    // http://en.wikipedia.org/wiki/UTF-8 
    $options = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'); 
     
    // A try/catch statement is a common method of error handling in object oriented code. 
    // First, PHP executes the code within the try block.  If, at any time, it encounters an 
    // error while executing that code, it stops immediately and jumps down to the 
    // catch block.  For more detailed information on exceptions and try/catch blocks: 
    // http://us2.php.net/manual/en/language.exceptions.php 
    try 
    { 
        // This statement opens a connect to your database using the PDO library 
        // PDO is designed to provide a flexible interface between PHP and many 
        // different types of database servers.  For more information on PDO: 
        // http://us2.php.net/manual/en/class.pdo.php 
        $db = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password, $options); 
    } 
    catch(PDOException $ex) 
    { 
        // If an error occurs while opening a connection to your database, it will 
        // be trapped here.  The script will output an error and stop executing. 
        // Note: On a production website, you should not output $ex->getMessage(). 
        // It may provide an attacker with helpful information about your code.  
        die("Failed to connect to the database: " . $ex->getMessage()); 
    } 
     
    // This statement configures PDO to throw an exception when it encounters 
    // an error.  This allows us to use try/catch blocks to trap database errors. 
    $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 
     
    // This statement PDO to return database rows from your database using an associative 
    // array.  This means the array will have string indexes, where the string value 
    // represents the name of the column in your database. 
    $db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC); 
     
    // This block of code is used to undo magic quotes.  Magic quotes are a terrible 
    // feature that was removed from PHP as of PHP 5.4.  However, older installations 
    // of PHP may still have magic quotes enabled and this code is necessary to 
    // prevent them from causing problems.  For more information on magic quotes: 
    // http://php.net/manual/en/security.magicquotes.php 
    if(function_exists('get_magic_quotes_gpc') && get_magic_quotes_gpc()) 
    { 
        function undo_magic_quotes_gpc(&$array) 
        { 
            foreach($array as &$value) 
            { 
                if(is_array($value)) 
                { 
                    undo_magic_quotes_gpc($value); 
                } 
                else 
                { 
                    $value = stripslashes($value); 
                } 
            } 
        } 
     
        undo_magic_quotes_gpc($_POST); 
        undo_magic_quotes_gpc($_GET); 
        undo_magic_quotes_gpc($_COOKIE); 
    } 
     
    // This tells the web browser that your content is encoded using UTF-8 
    // and that it should submit content back to you using UTF-8 
    header('Content-Type: text/html; charset=utf-8'); 
     
    // This initializes a session.  Sessions are used to store information about 
    // a visitor from one request to the next.  Unlike a cookie, the information is 
    // stored on the server-side and cannot be modified by the visitor.  However, 
    // note that in most cases sessions do still use cookies and require the visitor 
    // to have cookies enabled.  For more information about sessions: 
    // http://us.php.net/manual/en/book.session.php 
    session_start();

<?php 

    // First we execute our common code to connection to the database and start the session 
    require("common.php"); 
     
    // This variable will be used to re-display the user's username to them in the 
    // login form if they fail to enter the correct password.  It is initialized here 
    // to an empty value, which will be shown if the user has not submitted the form. 
    $submitted_username = ''; 
    
    // This if statement checks to determine whether the login form has been submitted 
    // If it has, then the login code is run, otherwise the form is displayed 
    if(!empty($_POST)) 
    { 
        // This query retreives the user's information from the database using 
        // their username. 
        $query = " 
            SELECT 
                id, 
                username, 
                password, 
                salt 
            FROM users 
            WHERE 
                username = :username 
        "; 
         
        $query_params = array( 
            ':username' => $_POST['username'] 
        ); 
         
        try 
        { 
            $stmt = $db->prepare($query); 
            $result = $stmt->execute($query_params); 
        } 
        catch(PDOException $ex) 
        { 
            // Note: On a production website, you should not output $ex->getMessage(). 
            // It may provide an attacker with helpful information about your code.  
            die("Failed to run query: " . $ex->getMessage()); 
        } 
         
        // This variable tells us whether the user has successfully logged in or not. 
        // We initialize it to false, assuming they have not. 
        // If we determine that they have entered the right details, then we switch it to true. 
        $login_ok = false; 
         
        $row = $stmt->fetch(); 
        if($row) 
        { 
            // Using the password submitted by the user and the salt stored in the database, 
            // we now check to see whether the passwords match by hashing the submitted password 
            // and comparing it to the hashed version already stored in the database. 
            $check_password = hash('sha256', $_POST['password'] . $row['salt']); 
            if($check_password === $row['password']) 
            { 
                // If they do, then we flip this to true 
                $login_ok = true; 
            } 
        } 
         
        // If the user logged in successfully, then we send them to the members page 
        // Otherwise, we display a login failed message and show the login form again 
        if($login_ok) 
        { 
            // Here I am preparing to store the $row array into the $_SESSION by 
            // removing the salt and password values from it.  Although $_SESSION is 
            // stored on the server-side, there is no reason to store sensitive values 
            // in it unless you have to.  Thus, it is best practice to remove these 
            // sensitive values first. 
            unset($row['salt']); 
            unset($row['password']); 
             
            // This stores the user's data into the session at the index 'user'. 
            // We will check this index on the members page to determine whether 
            // or not the user is logged in.  We can also use it to retrieve 
            // the user's details. 
            $_SESSION['user'] = $row; 
             
            // Redirect the user to the members page. 
            header("Location: brand.php"); 
            die("Redirecting to: brand.php"); 
        } 
        else 
        { 
            // Tell the user they failed 
            print("Login Failed."); 
             
            // Show them their username again so all they have to do is enter a new 
            // password.  The use of htmlentities prevents XSS attacks.  You should 
            // always use htmlentities on user submitted values before displaying them 
            // to any users (including the user that submitted them).  For more information: 
            // http://en.wikipedia.org/wiki/XSS_attack 
            $submitted_username = htmlentities($_POST['username'], ENT_QUOTES, 'UTF-8'); 
        } 
    } 
     
?> 
<?php include 'head.php' ?>
<!--TITLE-->
<title>Quiring | Login</title>
</head>

<!--START CONTENT-->
<body>
    <!--START WRAP-->
    <div class="row"><!--START .row-->    
        
        <!--START CONTENT-->
        <div class="three columns"></div><!--END .four columns-->
            <div class="six columns"><!--END .four columns-->
                <h1 class="login_h1">Quiring Login</h1>
                <img class="bigBump" src="images/logo_login.jpg" alt="Quiring Logo" />
            </div><!--END .four columns-->    
            <div class="three columns"></div><!--END .four columns-->
                <div class="row fclear">
                    <div class="three columns imgBump"></div><!--END .two columns-->
                    <div class="six columns imgBump">
                        <div id="login_container"><!--START #login_container-->
                            <form action="index.php" method="post"> 
                                <h3>Username:</h3> 
                                <input type="text" name="username" value="<?php echo $submitted_username; ?>" />
                                
                                <h3>Password:</h3>  
                                <input type="password" name="password" value="" /> 
                                <br />
                                <br />                 
                                <input class="button_login" type="submit" value="Login" />
                            </form>
                        </div><!--END #login_container-->
                    </div><!--END .eight columns-->
                    <div class="three columns imgBump"></div><!--END .two columns-->   
                </div><!--END .row-->
                <div class="twelve columns fclear"></div><!--END .twelve columns-->
            </div><!--END #QR_article-->
        <!--END CONTENT-->   
        </div><!--END .twelve columns-->
        
        </div><!--END #QR_wrap-->
    </div><!--END row-->
    <!--END WRAP-->
</body>
</html>