Hmmm... there is not much information about php errors in the apache log files (mod_php). There're even less in suphp logs. I'm not sure about fastcgi but I guess the same applies.
I think the best solution is to force a user to use the php hooks (
http://www.php.net/errorfunc) if required (for debugging on remote server).
Perhaps an easy (and dangerous) solution is to let the user upload a custom php.ini, but then we'll need to force some directives (like url extensions to fopen and include, which are quite used by php botnets, open_basedir etc).
Another solution would be to provide a (paid) debugging option where our vlogger(or what be) will be writing logs immediately to the user directory (and these could be picked up by an ispcp ajax tailer in the /tools/ directory). So you could open one browser for the php and another browser would be used for the ajax tailer (which would refresh on say 1-2 second intervals).
Being paid for, the user will probably disable it when he doesn't need it and there won't be so many open files on the server as now.
I'm just brainstorming some ideas ... feel free to comment.
ispcomm.