How about this.
1) When LSWS restarts, it spawns new php fastcgi group. Lsws would know at least the starting processid number of the new group.
2) Loop through current running processes and find all php processes with id lower than this number and send kill signal to them?
There is a loop-hole with this since lsws can kill long-running non-fastcgi spawned php proccesses such as scheduled cron jobs.