Locate the root-cause of defects, fast.
If you need to locate the source of a defect in complicated web-based PHP applications with lots of conditional includes (osCommerce and Zen-Cart are reasonably complex), use APD to create a profile dump.
Then filter the dump for lines containing "!" to produce a list of included (required) files in the order they were included.
On my Win98 dev box I use: find "!" dumpfile > trace_program.txt
I use the trace_program.txt to backtrack to the root-cause of defects appearing in the browser. Great for maintaining systems you did not build yourself.