profiling one part of an application

At the moment I'm testing the xdebug profiler.
I want to test only a portion of an application.

I use ' Tools-> Analysis Xdebug Profiler snapshot' .

How can I navigate to a part of the app.

How can I influence the output of the profiler?
For example:

-----------------------------
xdebug_start_profiler ();

... snip ...
my code
... snip ...

xdebug_stop_profiler ();
------------------------------

Sincerely yours

Stephan