How to delete pictures from server in php

public function delPic(){
    //获取前端传参 用户的uid
    $uid = I('uid');
    if(!$uid) $this->error('uid未获取');
    //获取url
    $url = M('Member')->where(array('uid'=>$uid))->getField('avatar');//$url = 'http://www.test.com/up/avatar/59b25bcfcaac6.jpg'
if(!$url) $this->error('数据库获取头像网址失败!');
    //获取$url有效字段（去掉网址）
    $str = substr($url, 20);//$str = 'up/avatar/59b25bcfcaac6.jpg'
    //file文件路径
    $filename = './'.$str;
    //删除
    if(file_exists($filename)){
        $info ='原头像删除成功';
        unlink($filename);
    }else{
        $info ='原头像没找到:'.$filename;
    }
    echo $info;
}