I would greatly appreciate something like "Find usage" function - something opposite to "Jump to definition".

It would throw up a list of files and lines where given Class/Method/Function/Property(/or even a variable) is used in given scope.
A node of usages in Code explorer would be nice too, though it would generate too much of a load, I suppose...

Very useful, indeed, but something that's harder to do with PHP than with more stringent languages. There are obvious limitations, like the use of an item through a variable variable. There are also harder-to-spot issues, like those involving scope visibility. However, I still agree that it'd be a useful feature as it'd work quite well in many common cases, and it'd be a great starting point for the manual process in more complicated scenarios.