targets are executed even if the target that depend on them is skipped using if/unless

My vote is that the documentation needs to be updated. Phing is basically a port/clone of Apache Ant and the Ant user manual is clear on the subject:

Important: the if and unless attributes only enable or disable the target to which they are attached. They do not control whether or not targets that a conditional target depends upon get executed. In fact, they do not even get evaluated until the target is about to be executed, and all its predecessors have already run.

http://ant.apache.org/manual/using.html#targets

This behavior is fairly fundamental to the circular reference and dependency resolution. As pointed out in the same Ant manual page, using a dependent task to set a property is also the only way to accomplish a compound if or unless conditional check.