Understanding PHP-FIG: Choosing Between PSR‑6 and PSR‑16 Caching Standards

PHP‑FIG Overview

PHP‑FIG (PHP Framework Interop Group) is a community‑driven consortium that publishes PHP Standard Recommendations (PSRs) to improve interoperability and portability across frameworks such as Symfony, Laravel, and Zend Framework.

PSR‑6 – Cache Interface

Goal

Provide a generic cache API that enables library authors to write cache‑aware code usable in any framework without writing framework‑specific adapters.

Key concepts

PSR‑6 introduces the notions of a cache pool (an abstraction of a backend such as Redis, APCu, Memcached) and cache items (objects representing individual key‑value entries). A pool manages a collection of items; each item implements CacheItemInterface.

Core interfaces

The Psr\Cache namespace defines four interfaces: CacheItemInterface – represents a single cache entry, provides get(), set(), isHit(), expiresAt(), expiresAfter(). CacheItemPoolInterface – manages a set of items, provides getItem(), getItems(), hasItem(), save(), saveDeferred(), commit(), deleteItem(), clear(). CacheException – base interface for all cache‑related exceptions. InvalidArgumentException – thrown when an invalid cache key is supplied.

Typical usage

$pool = new Psr6Implementation();
$item = $pool->getItem('foo');

if (!$item->isHit()) {
    // Fetch or compute the value
    $value = 'bar';
    $item->set($value);
    $pool->save($item);
} else {
    $value = $item->get();
}

return $value;

PSR‑6 supports deferred writes via saveDeferred() and batch commit with commit(), making it suitable for complex caching strategies.

PSR‑16 – Simple Cache

Goal

Offer a lightweight, easy‑to‑adopt key‑value cache API for straightforward use‑cases.

Key concepts

PSR‑16 does not expose cache pools or items; it treats the cache as a simple key‑value store.

Core interfaces

The Psr\SimpleCache namespace defines three interfaces: CacheInterface – primary API with methods get(), set(), delete(), clear(), has(), getMultiple(), setMultiple(), deleteMultiple() (all accept optional TTL). CacheException – base exception interface. InvalidArgumentException – thrown for invalid keys.

Typical usage

$cache = new Psr16Implementation();

if (!$cache->has('foo')) {
    // Fetch or compute the value
    $value = 'bar';
    $cache->set('foo', $value); // optional TTL can be passed as third argument
} else {
    $value = $cache->get('foo');
}

return $value;

When a default value is acceptable on a miss, the code can be reduced to a single call:

$cache = new Psr16Implementation();
return $cache->get('foo', 'bar');

PSR‑16 yields shorter code because it works with a single cache object and does not provide advanced features such as deferred writes.

Interoperability and Adoption

Both PSR‑6 and PSR‑16 declare a clear() method with identical signatures, allowing a single cache implementation to satisfy both interfaces on PHP 5.3.9+. Symfony 3.3+ ships adapters that convert a PSR‑6 CacheItemPoolInterface to a PSR‑16 CacheInterface and vice‑versa, facilitating gradual migration.