How to use watches right?

I'm maintaining a big ugly PHP-project, much giant classes with all public vars and stuff...

I wanted to add a few deep member variables of a class to the watchlist.

First I tried to use a $this ref in the watch, which broke when I left the method of the class:

$this->m_oMyVar->value

Then I got me a reference tree from a global object. I thought it's an absolut reference to the value, so it should be the same everywhere:

$application->...->...->...->m_oMyVar->value

Which worked but only till I left the request, when I entered a new request they showd just "error"

How could this feature possible help me, if it doesn't keep track of the variables I want?!