Decrease responsibility of a class

Recently I had the need to work with object-oriented sessions and created the following class:

<?php
/**
 * @author Mateus Demboski <[email protected]>
 * @link <https://github.com/mateusdemboski/PHPsessionManager>
 */

#namespace Demboski;

/**
 * Class Session
 */
class Session {

    /**
     * Verify if a PHP session already been started.
     *
     * @return bool
     */
    public static function isStarted(){
        return session_status() == PHP_SESSION_ACTIVE;
    }

    /**
     * Verify if a key exists on session.
     *
     * @param null $session_key
     * @return bool
     */
    public static function exists($session_key = null){
        return isset($_SESSION[$session_key]);
    }

    /**
     * Initialize session data. If session has already started in PHP the
     * script generate a exception.
     *
     * @return bool
     * @throws \Exception
     */
    public static function start(){
        if(self::isStarted())
            throw new \Exception('A session has already started in PHP!');

        return session_start();
    }

    /**
     * Destroys all data registered to a session.
     *
     * @return bool
     */
    public static function destroy(){
        return session_destroy();
    }

    /**
     * Get the value by key from current session.
     *
     * @param string|int $session_key
     * @param bool $unserialize
     * @return string|null
     */
    public static function get($session_key, $unserialize = false){
        if(!self::isStarted())
            throw new \BadFunctionCallException('A session in PHP need be started!');

        if(self::exists($session_key))

            if($unserialize)
                return unserialize(base64_decode($_SESSION[$session_key]));

            else
                return $_SESSION[$session_key];

        else return null;
    }

    /**
     * Assign a value from key in the current session.
     *
     * @param string $session_key
     * @param mixed $value
     * @param bool $serialize
     * @return bool
     */
    public static function set($session_key, $value, $serialize = false){
        if(!self::isStarted())
            throw new \BadFunctionCallException('A session in PHP need be started!');


        if($serialize) $value = base64_encode(serialize($value));

        $_SESSION[$session_key] = $value;

        return $_SESSION[$session_key] == $value;
    }

    /**
     * Delete an item from session by key.
     *
     * @param $session_key
     * @return bool
     */
    public static function delete($session_key){
        if(!self::isStarted())
            throw new \BadFunctionCallException('A session in PHP need be started!');

        $return = true;

        if(self::exists($session_key))
            unset($_SESSION[$session_key]);
        else
            $return = false;

        return $return;
    }

    /**
     * Destroys all data registered to a session and start a new session.
     *
     * @return bool
     */
    public static function clean(){
        if(!self::isStarted())
            throw new \BadFunctionCallException('A session in PHP need be started!');

        self::destroy();
        self::start();
        return true;
    }

    /**
     * Set the current session name.
     * The session name references the name of the session, which is
     * used in cookies and URLs (e.g. PHPSESSID).
     *
     * @param null $session_name
     * @return string
     */
    public static function setName($session_name = null){
        return session_name($session_name);
    }

    /**
     * Get the current session name.
     * The session name references the name of the session, which is
     * used in cookies and URLs (e.g. PHPSESSID).
     *
     * @return string
     */
    public static function getName(){
        return session_name();
    }

    /**
     * Set the current session id
     *
     * @param null $session_id
     * @return string
     */
    public static function setId($session_id = null){
        return session_id($session_id);
    }

    /**
     * Get the current session id
     *
     * @return string
     */
    public static function getId(){
        return session_id();
    }

    /**
     * Update the current session id with a newly generated one.
     *
     * @param bool $deleteOldSession
     * @return bool
     */
    public static function regenerateId($deleteOldSession = false){
        return session_regenerate_id($deleteOldSession);
    }

    /**
     * prevent the session hijacking
     *
     * @param string $session_key Default is "PREVENT_SESSION_HIJACKING".
     * @param array $hashAlgo Is the Name of selected hashing algorithm (e.g. "md5", "sha256", "haval160,4", etc..)
     * @param int|string $exit Default is 1.
     */
    public static function preventHijacking($session_key = 'PREVENT_SESSION_HIJACKING',
                                            $hashAlgo = 'md5',
                                            $exit = 1){
        $hashData = $_SERVER['HTTP_USER_AGENT'] . $_SERVER['REMOTE_ADDR'];
        $hash = hash($hashAlgo, $hashData);
        if(self::isStarted() && !self::exists($session_key))
            self::set($session_key, $hash );
        elseif(self::isStarted() || !self::exists($session_key) || self::get($session_key) != $hash ){
            exit($exit);
        }
    }
}

But it seems to me to have a lot of responsibilities, for example, the preventHijacking method is a security method for sessions and I do not seem to be in the right place, but at the same time I find it unnecessary to create a new class inside namespace for this method only.

What could be done in this case? How could this class be improved?