<form class="" action="" method="post">

                    <?php  login_user();?>

                <div class="form-group"><label for="">
                    username<input type="text" name="username" class="form-control"></label>
                </div>

                 <div class="form-group"><label for="password">
                    Password<input type="text" name="password" class="form-control"></label>
                </div>

                <div class="form-group">
                  <input type="submit" name="submit" class="btn btn-primary" >
                </div>

            </form>

function login_user(){

    if(isset($_POST['submit'])) {

        $username = $_POST['username'];
        $password = $_POST['password'];

$sqlQuery = "SELECT * FROM users WHERE username = \'{username\}' AND password = \'{password\}'";
$statement = $db->prepare($sqlQuery);
$statement->execute($username, $password);
$data = $statement->fetch();
if($data){
    header("location:admin");
}else {
    header("location:login.php");
}




    }
}