Unable to update table PDO update - php

Question

I'm attempting to update a table in a database using PDO. At present I'm submitting the form and getting nothing but a white screen, I've enabled all error reporting options and still nothing but a white screen.. I've been staring at the code for what feels like a lifetime and still can't resolve the issue. A push in the right direction would be much appreciated...Thanks
require('includes/config.php');
//if not logged in redirect to login page
if(!$user->is_logged_in()){ header('Location: login.php'); }
$signedin = $_SESSION['username'];
$sql = "UPDATE member SET firstname = :firstname,
lastname = :lastname,
username = :username,
email = :email,
age = :age,
country = :country
where username = $signedin";
$stmt = $db->prepare($sql);
$stmt->bindParam(':firstname', $_POST['firstname'], PDO::PARAM_STR);
$stmt->bindParam(':lastname', $_POST['$lastname'], PDO::PARAM_STR);
$stmt->bindParam(':username', $_POST['username'], PDO::PARAM_STR);
// use PARAM_STR although a number
$stmt->bindParam(':email', $_POST['email'], PDO::PARAM_STR);
$stmt->bindParam(':age', $_POST['age'], PDO::PARAM_STR);
$stmt->bindParam(':country', $_POST['country'], PDO::PARAM_INT);
$stmt= $db->execute($sql);
?>

Answer 1

The execute() function doesn't need the $sql (you provided that in prepare())
$stmt->execute();
Next, you should pass all your data into your prepared statement, otherwise you're defeating the purpose (which is maximum security). So let's remove
$sql = "UPDATE member SET firstname = :firstname,
lastname = :lastname,
username = :username,
email = :email,
age = :age,
country = :country
where username = :username";
//snip
$stmt->bindParam(':username', $_SESSION['username'], PDO::PARAM_STR);

Answer 2

You need the quotes in your where clause.
$sql = "UPDATE member SET firstname = :firstname,
lastname = :lastname,
username = :username,
email = :email,
age = :age,
country = :country
where username = '$signedin'";
Also, it's better to update by id since it's unique.