public function LoginUser($username, $password){
    // Check if user adn password matches an user in database
    if(isset($_POST['username'])){
        $username = mysqli_real_escape_string($this->db, $username);
        $password = mysqli_real_escape_string($this->db, $password);

        $sql = "SELECT * FROM users WHERE name='$username'";
        $result = mysqli_query($this->db, $sql) or die('Fel vid SQL-fråga - inloggning');

        if(mysqli_num_rows($result)){
            $row = mysqli_fetch_array($result);
            $stored_password = $row['password'];
            // check if hash_equals-function exists
            if(function_exists("hash_equals")) {
                //If excists
                if(hash_equals($stored_password, crypt($password, $stored_password))) {
                    // create session
                    header("location: user/loggedin.php");
                    $_SESSION['login'] = $username;
                }else{
                    echo '*WRONG!*';
                }
            }else{
                //if not exists, use alternate method
                if($stored_password == crypt($password, $stored_password)){
                    // create session that tells that we're logged in
                    header("location: ../user/loggedin.php");
                    $_SESSION['name'] = $username;
                }else{
                    echo 'WRONG!';
                }
            }
        }
    }
}

<?php
// call class User.php, LoginUser
$users = new User();
if(isset($_POST['loginButton'])){
    if($users->LoginUser($_POST['username'],     $_POST['password'])){
        //header("location: admin/admin.php");
    }else{
        echo 'wrong username or password';
    }
}

if(hash_equals($stored_password, crypt($password, $stored_password))) {
        // create session
                    header("location: user/loggedin.php");
        $_SESSION['login'] = $username;

    }else if($stored_password == crypt($password, $stored_password)){
        // create session that tells that we're logged in
        header("location: ../user/loggedin.php");
        $_SESSION['name'] = $username;
    }else{
        echo 'WRONG!';
    }

location: user/loggedin.php

location: ../user/loggedin.php