Variables storage

[manuel-rubio]

The original PHP stores the variables and adds a reference to them so, when an object, array or resource is assigned to a new variable, instead of a clone, it's copied a reference to the real data. In this way, the references are easier to handle internally and the weird behaviour for unset is understood better.

At this moment the implementation for unset is not compatible when we use it with objects because we're not destroying the object.

[manuel-rubio]

At this moment I think we need to check if ephp_mem is good enough.