致命错误：未定义的函数db（）（但它是）

So I have this pdo.php file that contains my Database connection. This is the function:

function db() {
    static $dbh;
    if(!isset($dbh)) {
        $dsn = sprintf('mysql:host=%s;dbname=%s', SHOP_DB_HOST, SHOP_DB_NAME);
        try {
            $dbh = new PDO($dsn, SHOP_DB_USER, SHOP_DB_PASSWORD);
        } catch(PDOException $e) {
            header('Status: 500 Internal Server Error');
            echo '<b>Error:</b> Database connection failed.';
            exit;
        }
        $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        $dbh->exec('SET NAMES utf8');
    }
    return $dbh;
}

And this is another file in which I need to use db():(It's detecting an ajax call and based on that, supposed to update information)

if(isset($_POST['updateTree'])) {
    $page_id = $_POST['page_id'];
    $parent_id = $_POST['parent_id'];
    $customer_id = $_SESSION['customer_id'];
    $stojkovQ = db()->prepare("
        UPDATE 
            cms_pages
        SET 
            topicId = :parent_id
        WHERE
            page_id = :page_id
        AND
            customer_id = :customer_id
    ");
    $stojkovQ->execute(array(
        'customer_id'    => $customer_id,
        'page_id'        => $page_id,
        'topicId'        => $parent_id
    ));
    echo 'updated';
}

So after after getting an "Fatal Error: Call to undefined function db()" even tho it's included in my header.php file(containing all the includes and config information for my site). If I try to include it again just above my query it yields another error(Cannot redeclare db())

Any lead towards the resolving of this problem will be of great help.