1、php如何实现session

执行php脚本，session_start()会从php.ini中读取配置项，将生成的唯一值sessionID保存文件放到配置项中的保存路径和地点。并通过HTTP协议返回响应消息头setCookie
Cookie名=Cookie值发送给客户端。

客户端接受到Set-Cookie,将cookie值写入cookie文件

在接下来的访问中，客户端会携带cookie访问浏览器，浏览器接受到cookie值，生成http请求头将包含的COOKIE发送给Php,php识别cookie值，从保存路径中找对应的文件。

找到文件之后，检验是否在有效期内，在有效期内就读取文件，不再有效期内就清空文件

2、自己实现session需要考虑的几个问题

1)、生成唯一值的算法

2)、将session存到文件里还是存到redis还是memcache，应该存什么数据

3)、接受cookie值，从保存位置找到对应的文件或数据

4)、session垃圾回收机制，删除session文件和数据

5)、分布式的话，session同步问题

3、分析一下laravel如何实现session.

vendor/laravel/framework/src/Illuminate/Contracts定义都是接口类（契约）。
看一下session接口定义的是什么？

<?php

namespace Illuminate\Contracts\Session;

interface Session
{
    public function getName();//获得session名字

    public function getId();//获得当前sessionID

    public function setId($id);//修改sessionID

    public function start();//启动session,从配置文件中读取数据

    public function save();//将session数据保存

    public function all();//获得所有的session

    public function exists($key);//检查session名称是否存在

    public function has($key);//检查session名称是否存在和不为空

    public function get($key, $default = null);//通过session名称获取session值

    public function put($key, $value = null);//将session名称和session值存入session文件或数据库

    public function token(); //获得csrf token的值
 
    public function remove($key);//根据session名称删除session信息

    public function forget($keys);//根据session名称删除session信息

    public function flush();//清空所有的session内容

    public function migrate($destroy = false); //为session会话创建一个新的session会话

    public function isStarted();//确定会话是否启动

    public function previousUrl();//从会话中获取session的url

    public function setPreviousUrl($url);//设置previous的url存入session

    public function getHandler();//获得session处理实例

    public function handlerNeedsRequest();//确定会话程序是否需要请求

    public function setRequestOnHandler($request);//在处理请求实例上设置请求
}

实现接口类的具体实现类只有一个vendor/laravel/framework/src/Illuminate/Session/Store.php

<?php

namespace Illuminate\Session;

use Closure;
use stdClass;
use Illuminate\Support\Arr;
use Illuminate\Support\Str;
use SessionHandlerInterface;
use Illuminate\Contracts\Session\Session;

class Store implements Session
{
    protected $id;

    protected $name;

    protected $attributes = [];

    protected $handler;

    protected $started = false;

    public function __construct($name, SessionHandlerInterface $handler, $id = null)
    {
        $this->setId($id);
        $this->name = $name;
        $this->handler = $handler;
    }

    public function start()
    {
        $this->loadSession();

        if (! $this->has('_token')) {
            $this->regenerateToken();
        }

        return $this->started = true;
    }

    protected function loadSession()
    {
        $this->attributes = array_merge($this->attributes, $this->readFromHandler());
    }

    protected function readFromHandler()
    {
        if ($data = $this->handler->read($this->getId())) {
            $data = @unserialize($this->prepareForUnserialize($data));

            if ($data !== false && ! is_null($data) && is_array($data)) {
                return $data;
            }
        }

        return [];
    }

    protected function prepareForUnserialize($data)
    {
        return $data;
    }


    public function save()
    {
        $this->ageFlashData();

        $this->handler->write($this->getId(), $this->prepareForStorage(
            serialize($this->attributes)
        ));

        $this->started = false;
    }

    protected function prepareForStorage($data)
    {
        return $data;
    }


    public function ageFlashData()
    {
        $this->forget($this->get('_flash.old', []));

        $this->put('_flash.old', $this->get('_flash.new', []));

        $this->put('_flash.new', []);
    }


    public function all()
    {
        return $this->attributes;
    }


    public function exists($key)
    {
        $placeholder = new stdClass;

        return ! collect(is_array($key) ? $key : func_get_args())->contains(function ($key) use ($placeholder) {
            return $this->get($key, $placeholder) === $placeholder;
        });
    }


    public function has($key)
    {
        return ! collect(is_array($key) ? $key : func_get_args())->contains(function ($key) {
            return is_null($this->get($key));
        });
    }

    public function get($key, $default = null)
    {
        return Arr::get($this->attributes, $key, $default);
    }


    public function pull($key, $default = null)
    {
        return Arr::pull($this->attributes, $key, $default);
    }

    public function hasOldInput($key = null)
    {
        $old = $this->getOldInput($key);

        return is_null($key) ? count($old) > 0 : ! is_null($old);
    }

    public function getOldInput($key = null, $default = null)
    {
        return Arr::get($this->get('_old_input', []), $key, $default);
    }


    public function replace(array $attributes)
    {
        $this->put($attributes);
    }

    public function put($key, $value = null)
    {
        if (! is_array($key)) {
            $key = [$key => $value];
        }

        foreach ($key as $arrayKey => $arrayValue) {
            Arr::set($this->attributes, $arrayKey, $arrayValue);
        }
    }


    public function remember($key, Closure $callback)
    {
        if (! is_null($value = $this->get($key))) {
            return $value;
        }

        return tap($callback(), function ($value) use ($key) {
            $this->put($key, $value);
        });
    }


    public function push($key, $value)
    {
        $array = $this->get($key, []);

        $array[] = $value;

        $this->put($key, $array);
    }


    public function increment($key, $amount = 1)
    {
        $this->put($key, $value = $this->get($key, 0) + $amount);

        return $value;
    }


    public function decrement($key, $amount = 1)
    {
        return $this->increment($key, $amount * -1);
    }


    public function flash(string $key, $value = true)
    {
        $this->put($key, $value);

        $this->push('_flash.new', $key);

        $this->removeFromOldFlashData([$key]);
    }

    public function now($key, $value)
    {
        $this->put($key, $value);

        $this->push('_flash.old', $key);
    }

    public function reflash()
    {
        $this->mergeNewFlashes($this->get('_flash.old', []));

        $this->put('_flash.old', []);
    }

    public function keep($keys = null)
    {
        $this->mergeNewFlashes($keys = is_array($keys) ? $keys : func_get_args());

        $this->removeFromOldFlashData($keys);
    }


    protected function mergeNewFlashes(array $keys)
    {
        $values = array_unique(array_merge($this->get('_flash.new', []), $keys));

        $this->put('_flash.new', $values);
    }

    protected function removeFromOldFlashData(array $keys)
    {
        $this->put('_flash.old', array_diff($this->get('_flash.old', []), $keys));
    }


    public function flashInput(array $value)
    {
        $this->flash('_old_input', $value);
    }


    public function remove($key)
    {
        return Arr::pull($this->attributes, $key);
    }

    public function forget($keys)
    {
        Arr::forget($this->attributes, $keys);
    }

    public function flush()
    {
        $this->attributes = [];
    }

    public function invalidate()
    {
        $this->flush();

        return $this->migrate(true);
    }

    public function regenerate($destroy = false)
    {
        return tap($this->migrate($destroy), function () {
            $this->regenerateToken();
        });
    }

    public function migrate($destroy = false)
    {
        if ($destroy) {
            $this->handler->destroy($this->getId());
        }

        $this->setExists(false);

        $this->setId($this->generateSessionId());

        return true;
    }

    public function isStarted()
    {
        return $this->started;
    }


    public function getName()
    {
        return $this->name;
    }


    public function setName($name)
    {
        $this->name = $name;
    }

    public function getId()
    {
        return $this->id;
    }

    public function setId($id)
    {
        $this->id = $this->isValidId($id) ? $id : $this->generateSessionId();
    }

    public function isValidId($id)
    {
        return is_string($id) && ctype_alnum($id) && strlen($id) === 40;
    }

    protected function generateSessionId()
    {
        return Str::random(40);
    }

    public function setExists($value)
    {
        if ($this->handler instanceof ExistenceAwareInterface) {
            $this->handler->setExists($value);
        }
    }


    public function token()
    {
        return $this->get('_token');
    }


    public function regenerateToken()
    {
        $this->put('_token', Str::random(40));
    }


    public function previousUrl()
    {
        return $this->get('_previous.url');
    }


    public function setPreviousUrl($url)
    {
        $this->put('_previous.url', $url);
    }


    public function getHandler()
    {
        return $this->handler;
    }

    public function handlerNeedsRequest()
    {
        return $this->handler instanceof CookieSessionHandler;
    }

    public function setRequestOnHandler($request)
    {
        if ($this->handlerNeedsRequest()) {
            $this->handler->setRequest($request);
        }
    }
}