apcu_store can be used to store, apcu_fetch to retrieve values:

$key = 'Hello';
$value = 'World';
apcu_store($key, $value);
print(apcu_fetch('Hello')); // 'World'

apcu_cache_info provides information about the store and its entries:

print_r(apcu_cache_info());

Note that invoking apcu_cache_info() without limit will return the complete data currently stored.
To only get the meta data, use apcu_cache_info(true).
To get information about certain cache entries better use APCUIterator.

The APCUIterator allows to iterate over entries in the cache:

foreach (new APCUIterator() as $entry) {
    print_r($entry);
}

The iterator can be initialized with an optional regular expression to select only entries with matching keys:

foreach (new APCUIterator($regex) as $entry) {
    print_r($entry);
}

Information about a single cache entry can be obtained via:

$key = '…';
$regex = '(^' . preg_quote($key) . '$)';
print_r((new APCUIterator($regex))->current());