Simple file cache static class

FileCache.php

<?php
define('DS', DIRECTORY_SEPARATOR);
class FileCache 
{
    public static $options = array(
        'folder' => 'tmp',
        'prefix' => 'havaianas_cache_',
        'expires' => '+20 minutes'
    );

    public static $directory;

    private static function init()
    {
        self::$directory = dirname(__FILE__) . DS . self::$options['folder'];
    }

    private static function getFilePath($key)
    {
        return self::$directory . DS . self::$options['prefix'] . $key;
    }

    public static function read($key)
    {
        self::init();
        if (!file_exists(self::getFilePath($key))) {
            return null;
        }

        $cached = unserialize(file_get_contents(self::getFilePath($key)));
        
        if ($cached['expires'] <= strtotime('now')) {
            unlink(self::getFilePath($key));
            return null;
        }

        return $cached['content'];
    }

    public static function write($key, $object)
    {
        self::init();
        $cached = array(
            'expires' => strtotime(self::$options['expires']),
            'content' => $object
        );
        $status = file_put_contents(self::getFilePath($key), serialize($cached));
        return $status !== false;
    }
}