<?php echo form_open('login'); ?>
            <div class="form-group">
                <div class="input-group">
                    <div class="input-group-prepend">
                        <div class="input-group-text"><i class="fas fa-user"></i></div>
                    </div>
                    <input type="text" class="form-control" name="username" placeholder="Usuario" required autofocus>
                </div>
            </div>

            <div class="form-group">
                <div class="input-group">
                    <div class="input-group-prepend">
                        <div class="input-group-text"><i class="fas fa-key"></i></div>
                    </div>
                    <input type="password" class="form-control" name="password" placeholder="Contraseña" required>
                </div>
            </div>

            <div class="form-group">
                <button type="submit" class="btn btn-primary btn-block">Entrar</button>
            </div>

            <?php echo form_close(); ?>

public function index()
{
    // Extraer campos necesarios
    $username = $this->input->post('username');
    $password = $this->input->post('password');

    $this->output->enable_profiler();

    // Buscar usuario.
    $this->load->model('users_model');
    $user = $this->users_model->getUserByUsername($username);