[Monarobase]

how can they communicate with each other (account?), can you give example?

Here's an example :
With APC and PHP-FPM, APC is launched at the startup of PHP when it is owned by root so when PHP-FPM forks an instance of PHP this instance accesses a global instance of APC.

If the unix user : UserA runs

apc_store ('foo', 'hello world');

Then unix user: UserB runs :

print apc_fetch('foo');

It will output 'hello world'

If unix user UserB then runs :

apc_store('foo', 'Goodbye world');

UserA expects the value to be Hello world but when he runs :

apc_fetch('foo');

it outputs 'Goodbye World'