[regularfry]

The problem is that the issues created by out-of-sync code that happens to get loaded in the wrong order can be an absolute nightmare to debug. You'll have transient, unreproducible, potentially data-destroying bugs which vary with each release. Some releases you might get lucky, some not. If you don't think about atomicity of the deployment, you can chase your tail for days trying to figure out what went wrong.

That being said, this strikes me as more of a pain under the traditional PHP model, where reloading code from disc per request is normal, than for something like Rails which loads everything into memory once at launch.

[martinaglv]

With APC (and stat disabled) PHP behaves in exactly the same way. Bytecode is kept in memory between requests and you can then safely push a new version of your directory tree. All that is required is to flush the cache to have the new version go live.