$host = 'xxxx';
$dbname = 'xxxx';
$username = 'xxxx';
$password = 'xxxx.';

try {
    $conn = new PDO("mysql:host=$host; dbname=$dbname;", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    echo "$e->getMessage()";
}

session_start();
include 'config.php';

    function getAddress(){
       $output = "";
       if (isset($_SESSION['userid'])){
            $userid = $_SESSION['userid'];
            try {
                $stmt = $conn->prepare('SELECT address FROM users WHERE id = :userid');
                $stmt->bindParam(':userid', $userid);
                $stmt->execute();
                $result = $stmt -> fetch();
                if(empty($result)){
                    $output = "Your information cannot be retrieved";
                } else {
                    $output = $result['address'];
                }
            } catch (PDOException $e) {
                $output = $e->getMessage();
            }
            $conn = null;
            } else {
            $output = "An error has occured, please try again";
            }
        return $output;
    }

<?php
session_start();
include 'functions.php';
?>
...
<p><?php echo getAddress();?></p>
...