When specifying a ttl (Time-To-Live), you are allowed to use negative values. This causes a stored entry to be invalidated immediately, but note that it will not physically be removed until you read (eg. apc_fetch or apc_exists) it:
apc_store('testKey', 'testValue', -1); $test = apc_fetch('testKey'); ?>
Although you could mimic apc_delete with a negative TTL (like when dealing with cookies), note that the difference is that apc_delete actually physically removes the entry.
This example comes in handy when you want to unit-test a class that uses APC. It's faster to use a negative TTL than a positive in combination with a sleep call to test whether the cache entry has actually been deleted. Other than that I can't think of any situations in which you could use this example, but I'm simply pointing out that doing this will not generate Exceptions or errors.