You've not said what you are rying to acheive with this monitoring / testing. Mod_php (like all apache modules) is a shared library, hence there is no measurable response time between apache and mod_php.
Measuring request response time on the webserver is a good idea- it gives some insight into into infrastrcture problems but tells you very little about what performance the client is experiencing (at least it shouldn't). Hint: you should be logging and analysing %D. This is where RUM comes in - although this is hard to implement on a test environment.
It is possible to project data from PHP into Apache using apache_note() (e.g. for logging memory_get_peak_usage()).
Other things you should plumb in:
- an opcode cache manager
- SAR
- mod_status
...and capture the data.