As of the V3 the bundle is absolutely not compatible with previous versions.
To ensure you the smoothest migration possible, please check the migration guide in the Resources/Docs directory.
One of the biggest change is the Phpfastcache's dependency which is not set to the v7 which it not backward compatible at all.

composer require phpfastcache/phpfastcache-bundle

# PhpFastCache configuration
phpfastcache:
    twig_driver: "filecache" # This option must be a valid declared driver, in our example: "filecache"
    twig_block_debug: false # This option will wrap CACHE/ENDCACHE blocks with block debug as HTML comment
    drivers:
        filecache:
            type: Files
            parameters:
                path: "%kernel.cache_dir%/phpfastcache/"

• This step can be skipped using Symfony recipes.

Caching data in your controller:

public function indexAction(Request $request, Phpfastcache $phpfastcache)
{
    $cache = $phpfastcache->get('filecache');
    $item = $cache->getItem('myAppData');
    
    if (!$item->isHit() || $item->get() === null) {
        $item->set('Wy app has now superpowers !!')->expiresAfter(3600);//1 hour
        $cache->save($item);
    } 
     
    // replace this example code with whatever you need
    return $this->render('default/index.html.twig', [
        'myAppData' => $item->get(),
        'base_dir' => realpath($this->getParameter('kernel.root_dir').'/..'),
    ]);
}

Or in your template:

<div>
    {#
     * 'myrandom6' Is your cache key identifier, must be unique
     * 300 Is the time to live (TTL) before the cache expires and get regenerated
    #}
    {% cache 'myrandom6' 300 %}
        <textarea>
            <!-- Some heavy stuff like Doctrine Lazy Entities -->
            {% for i in 1..1000 %}{{ random() }}{% endfor %}
        </textarea>
    {% endcache %}
</div>

As of the V3, some command-line tools were introduced, mostly for CRUD-like operations.

php bin/console phpfastcache:get filecache cacheKey

This will display the content of a cache item if it eventually exists.

php bin/console phpfastcache:get filecache cacheKey '{"a": 14}' 300 -a 1

This will set the content of a cache item.
The TTL (300), in seconds, is optional and take the default value filled in your configuration file.
The auto-type-cast option "-a" (enabled by default) will let allows you to automatically type cast your variable:

• false and true will be respectively converted to boolean.
• 1337 and 1337.666 will be respectively be converted to integer or float.
• null will be converted to null.
• {"a": 14} will be converted to an associative array using a JSON detection
• a regular string will remains unchanged and stays a string

You can obviously disable this behavior by turning off the auto-type-cast option: -a 0

php bin/console phpfastcache:del filecache cacheKey

This will delete the specified cache item.

php bin/console phpfastcache:clear filecache
# OR to clear every caches:
php bin/console phpfastcache:clear

This will clear a single cache instance if specified or all the configured cache instances otherwise.

As of the V3 there's a new, easier and cleaner way to setup HTTP cache to decrease your server bandwidth along with your CPU load: Cacheable Responses. And it's pretty easy to implement:

    /**
     * @Route("/cached", name="cached")
     */
    public function cachedAction(Phpfastcache $phpfastcache, Request $request): Response
    {
        return (new CacheableResponse($phpfastcache->get('filecache'), $request))->getResponse('cache_key', 3600, function () {
            return new Response('Random bytes: ' . \random_bytes(255));
        });
    }

CacheableResponse is provided by \Phpfastcache\Bundle\Response\CacheableResponse. This class will handle responses headers (cache-control, etag, etc...) and http status (304 Not modified).

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