Yii Cache::get() cannot distinguish cached false from missing key

Answer

In Yii 2, call Cache::has() before Cache::get() to tell a cached false from a missing key. If you are on Yii 1.x, or want a single-call pattern that works across adapters, pass a unique sentinel as the default value to get() instead.

Why false is ambiguous

The base Cache::get() contract returns false both when a key is absent and when the stored value is boolean false. This applies to FileCache, DbCache, MemCache, and RedisCache because the ambiguity is in the base method, not the adapter. getOrSet() avoids unnecessary recomputation on a miss, but it still returns false and does not reveal whether the value came from the cache.

Yii 2: existence check first

if ($cache->has('feature_flag')) {
    $value = $cache->get('feature_flag');
} else {
    $value = computeFeatureFlag();
    $cache->set('feature_flag', $value);
}

A key can expire between has() and get() under heavy concurrency; if that matters, use the sentinel pattern below.

Yii 1.x and cross-adapter: sentinel default

define('CACHE_MISS', 'APP_CACHE_MISS_SENTINEL');

$raw = $cache->get('feature_flag', CACHE_MISS);
if ($raw === CACHE_MISS) {
    $value = computeFeatureFlag();
    $cache->set('feature_flag', $value);
} else {
    $value = $raw; // may legitimately be false, null, 0, or ''
}

Yii 1.x does not expose Cache::has() consistently, so this sentinel default is the portable workaround. The sentinel must be a value your application never stores under the same key.

Verification

1. $cache->set('test', false);
2. Assert $cache->has('test') is true, then $cache->get('test') returns false.
3. $cache->delete('test');
4. Assert $cache->has('test') is false, and $cache->get('test', CACHE_MISS) returns CACHE_MISS.
5. Repeat with null, 0, and an empty string to confirm the sentinel does not collide.

Missing diagnostic detail: which Yii version are you using? Cache::has() is the straightforward answer for Yii 2; Yii 1.x requires the sentinel workaround.