<?php
// Authentication Check
if(!isset($_SESSION['user']['is_logged_in'] || $_SESSION['user']['is_logged_in'] !== true) )
{
    if( isset($_POST['username']) && isset($_POST['password']) )
    {
        // Query database for valid user details
        $dbh->query("SELECT password FROM users WHERE username='{$_POST['username']}' LIMIT 1");

        $password = $dbh->fetch(PDO::FETCH_ASSOC);

        // Verify user password is valid
        if( $password['password'] !== sha1($_POST['password']) )
        {
            // User validation fails
        }
        else
        {
            // Set user session data
            $_SESSION['user']['is_logged_in'] = true;
            $_SESSION['user']['username'] = $_POST['username'];
        }
    }
}