PHP文件缓存类

<?php
    /**
     * @desc 文件缓存
     */
    class Cache{
        const C_FILE = '/Runtime/';
        private $dir = '';
        const EXT = '.tpl';
        private $filename = '';
        public function __construct($dir = ''){
            $this->dir = $dir;
            
        }
        /**
         *    @desc 设置文件缓存
         *    @param string $key 文件名 
         *    @param unkonw $data 缓存数据
         *    @param int    $expire 过期时间
         */
        public function set($key,$data,$expire = 0){
            $this->filename =     dirname(__FILE__).self::C_FILE.$this->dir.$key.self::EXT;
            if(file_exists($this->filename)){
                $res = $this->get($key);
                if(md5($res) == md5(json_encode($data) ) ){
                    return true;
                }
            }
            if(!is_dir(dirname($this->filename))){
                mkdir(dirname($this->filename),0777);
            }
            
            $source = fopen($this->filename,'w+');
            fwrite($source,json_encode($data));
            fclose($source);
        }
        
        /**
         *    @desc 获取文件
         *  @param string $key 文件名
         */
        public function get($key){
            //$filename = dirname(__FILE__).self::C_FILE.$this->dir.$key.self::EXT;
            if(!file_exists($this->filename)){
                return '缓存文件已经不存在';
            }else{
                $res = file_get_contents($this->filename);
            }
            return $res;
        }
        /**
         *    @desc 删除文件
         *  @param string $key 文件名
         */
        public function del($key){
            unlink($this->filename);
        }
        
    }
    
    $data = array('name'=>'song','age'=>20,'sex'=>'man','favority'=>array('apple','banana'));
    $cache = new Cache();
    $cache->set('cache',$data);
    //$cache->get('cache');
    //$cache->del('cache');