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Check this example: https://github.com/phalcon/invo/blob/master/app/controllers/SessionController.php

This is a very minimalist implementation of what you want:

users.txt

foo;hashedPassword
bar;hashedPassword

Auth controller:

class SessionController extends ControllerBase
{
    private function _lookupUser($user, $password)
    {
        $found = false;
        $fh = fopen('users.txt', 'r');
        flock($fh, LOCK_SH);
        while($row = fgetcsv($fh, 512, ';')) {
            if($row[0] == $user && $row[1] == $this->security->hash($password)) {
                $found = true;
                break;
            }
        }
        flock($fh, LOCK_UN);
        fclose($fh);
        return $found;
    }
    private function _registerSession(array $user)
    {
        $this->session->set('auth', $user);
    }
    public function startAction()
    {
        if ($this->request->isPost()) {
            $user = $this->request->getPost('user');
            $password = $this->request->getPost('password');
            if(!$this->_lookupUser($user, $password)) {
                $this->_registerSession(array('user'=>$user));
                $this->flash->success('Welcome ' . $user);
                return $this->dispatcher->forward(
                    [
                        "controller" => "invoices",
                        "action"     => "index",
                    ]
                );
            }
            $this->flash->error('Wrong user/password');
        }
        return $this->dispatcher->forward(
            [
                "controller" => "session",
                "action"     => "index",
            ]
        );
    }
}

Then in any controller, just access $this->session->get("user")