code: public function exists($key)
      {
          $key = $this->buildKey($key);
  
          return $this->useApcu ? apcu_exists($key) : apc_exists($key);
      }
docs: Checks whether a specified key exists in the cache.
  This can be faster than getting the value from the cache if the data is big.
  Note that this method does not check whether the dependency associated
  with the cached data, if there is any, has changed. So a call to [[get]]
  may return false while exists returns true.
  @param mixed $key a key identifying the cached value. This can be a simple string or
  a complex data structure consisting of factors representing the key.
  @return bool true if a value exists in cache, false if the value is not in the cache or expired.