I've recently been working with AJAX and SOAP which both result in multiple sessions running concurrently.
For example if you have an AJAX progress bar for a large file upload one session is uploading the file while a series of shorter sessions check on progress and update the bar.
How to track both parts of this cleanly has had me scratching my head a bit - but the solution is fiendishly simple.
It turns out that there is a buried configuration setting in Eclipse (my IDE of choice) which allows you to debug multiple sessions.
in Eclipse go to:
Window | Preferences ... | PHP | Debug | Installed Debuggers | XDebug | Configure
Then check the box that says 'Use Multisession'
Lo and Behold the debug pane now has a multisession manager and allows you to see what is happening on both sessions.
see also
http://dev.eclipse.org/newslists/news.eclipse.tools.pdt/msg01476.html [1]