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<?php
$redis = new Predis\Client;
$handler = new RedisSession($redis);

session_set_save_handler($handler);
session_start();

// Use the $_SESSION superglobal as normal
$_SESSION['foo'] = 'bar';

<?php

use Predis\Client;

class RedisSession implements SessionHandlerInterface {
    private $redis;
    private $keyPrefix;
    private $maxLifetime;

    /**
     * Let's get this redis party started.
     * 
     * @param Predis\Client $redis     The Predis Client
     * @param string        $keyPrefix The Redis Key prefix
     */
    public function __construct(Client $redis, $keyPrefix = 'session:') {
        $this->redis = $redis;
        $this->keyPrefix = $keyPrefix;

        // We want to just use
        $this->maxLifetime = ini_get('session.gc_maxlifetime');
    }
 
    /**
     * We don't need to do anything extra to initialize the session since
     * we get the Redis connection in the constructor.
     *
     * @param  string $savePath The path where to storethe session.
     * @param  string $name     The session name.
     */
    public function open($savePath, $name) { }

    /**
     * Since we use Redis EXPIRES, we don't need to do any special garbage
     * collecting.
     *
     * @param  string $maxLifetime The max lifetime of a session.
     */
    public function gc($maxLifetime) { }
 
    /**
     * Close the current session by disconnecting from Redis.
     */
    public function close() {
        // This will force Predis to disconnect.
        unset($this->redis);
    }
 
    /**
     * Destroys the session by deleting the key from Redis.
     * 
     * @param  string $sessionId The session id.
     */
    public function destroy($sessionId) {
        $this->redis->del($this->keyPrefix.$sessionId);
    }

    /**
     * Read the session data from Redis.
     * 
     * @param  string $sessionId The session id.
     * @return string            The serialized session data.
     */
    public function read($sessionId) {
        $sessionId = $this->keyPrefix.$sessionId;
        $sessionData = $this->redis->get($sessionId);

        // Refresh the Expire
        $this->redis->expire($sessionId, $this->maxLifetime);
        return $sessionData;
    }
 
    /**
     * Write the serialized session data to Redis. This also sets
     * the Redis key EXPIRES time so we don't have to rely on the
     * PHP gc.
     * 
     * @param  string $sessionId   The session id.
     * @param  string $sessionData The serialized session data.
     */
    public function write($sessionId, $sessionData) {
        $sessionId = $this->keyPrefix.$sessionId;

        // Write the session data to Redis.
        $this->redis->set($sessionId, $sessionData);

        // Set the expire so we don't have to rely on PHP's gc.
        $this->redis->expire($sessionId, $this->maxLifetime);
    }

}