incapsulation

<?php

class FileSystem {
    public function store($file)
    {
        $this->checkFreeSpace();
        $this->storeFile($file);
    }
    private function checkFreeSpace()
    {
        if (free_disk_space() == 0) {
            throw new Exception("Not enought free space.");
        }
    }
    private function storeFile($file)
    {
        // Real code for store file
    }
}