<?php
 session_start();
 require('../model/database.php');
 require('../model/admin_db.php');

 $action = filter_input(INPUT_POST, 'action');

 if ($action === NULL) {

   $action = filter_input(INPUT_GET, 'action');

 if ($action === NULL) {
    $action = '';
     }
 }

 if(!isset($_SESSION['is_valid_admin'])){

   $action = 'get_admin';
 }
  switch($action){

   case 'get_admin':
    $username = filter_input(INPUT_POST, 'username');
    $password = filter_input(INPUT_POST, 'password');
            echo $username;
            echo $password;
            echo is_valid_admin_login($username, $password);

 if (is_valid_admin_login($username, $password)){
        $_SESSION['is_valid_admin']=true;
        include ('admin_menu.php');
        }

 else{
            $login_message = 'you must login to view this page.';
            include ('adminlogin.php');
        }

 include ('adminlogin.php');
 break;

<?php
function is_valid_admin_login($username, $password) {
global $db;
$password = sha1($username . $password);
$query = 'SELECT username FROM administrators
            WHERE username = :username
                            AND password = :password';

$statement = $db->prepare($query);
$statement->bindValue(':username', $username);
$statement->bindValue(':password', $password);
$statement->execute();
$valid = ($statement->rowCount() == 1);
$statement->closeCursor();
return $valid;
 }
?>