apc_add() versus apc_store()

The only difference between those two functions is that apc_add() will not overwrite an existing entry.
apc_store(), on the other hand, will overwrite an existing entry.

So, which one should your use, between apc_add() and apc_store() ?
Well, it all depends on your needs : do you want an existing entry to be overwritten, or not ?

In addition to what @Pascal MARTIN writes, there is a major practical difference between them.

In a heavily concurrent environment, e.g. when using apc for user objects in apache, the use of apc_store might lead to apc time bomb (the link is from 2007 but still relevant!), while apc_add mitigates this issue.

See also: https://serverfault.com/questions/342295/apc-keeps-crashing