Q:

what is the easiest way for opcode caching with PHP/Apache?

I was thinking to use opcode caching for performance profit
what is the easiest way for opcode caching with PHP/Apache ?
and what are the performance improvements ?
I have read about xDebug but I was wondering if there are more options ?

A:

I use the APC extension as an opcode cache on both my personnal server, and on the servers we are using at work -- and I've almost never run into any kind of trouble with it.

Installation is pretty easy : depending on your Linux distribution, you might want to use something like :
sudo aptitude install php5-apc

But you'd not necessarily have the last stable version... And I generally prefer using :
sudo pecl install apc

Which will fetch the last version from PECL and compile it (Note : you'll probably need to installed a package called php5-dev).
You'll then have to configure it ; see Runtime Configuration for the directives you can configure.

The performance improvements can vary depending on your server/application, but, on a server that only serves PHP pages (i.e. if your DB is on another machine) you might see a drop in CPU load that's quite important (I've seen CPU load go from 80% to 40-50% on a server, once)

Xdebug is not related to opcode caching nor performance : as its name indicates, it's useful for debugging.
And Xdebug should not be installed on a production server : it can be a pain, when it comes to performances -- I suppose it's because it "hooks" into PHP and add lots of stuff, like logging many informations useful for debugging ; which means more calculations to do -- which takes time and CPU.

