Very simple and basic Session Login Class

Authentication.php

<?php

class Authentication
{
    /**
     * hashed with password_hash and self::HASH_ALGORITHM
     */
    protected $users = [
        'admin' => '$2y$10$ea80bTD/1qmS.HFJbSL2luYYXYhYVOkUq7J6obihEjg/p0EHzoKPm', 
    ];

    const HASH_ALGORITHM = PASSWORD_BCRYPT;

    public function __construct()
    {
        @session_start();
    }

    public function login(string $username, string $password): bool
    {
        if (!isset($this->users[$username])) {
            return false;
        }

        if (!password_verify($this->users[$username], $password)) {
            return false;
        }

        $_SESSION['is_logged_in'] = true;
        
        return true;
    }

    public function logout() 
    {
        $_SESSION['is_logged_in'] = false;
    }

    public function isAuthenticated() 
    {
        return $_SESSION['is_logged_in'];
    }

    public function check()
    {
        return $this->isAuthenticated();
    }
}