don't forget that in posix systems you can use /proc/$process_id/
files:
auxv
cmdline
cwd
environ
exe
fd
maps
mem
mounts
root
stat
statm
status
task
wchan
with it you can make somethings like:
$PPID=getmypid();
$pid=pcntl_fork();
if ($pid==0){
while(1){
if (strpos(file_get_contents("/proc/$PPID/cmdline"),'php')===false) echo "parent pid die";
}
}else{
// forked
}
with this you can use some libs that block signal or some bad signal handling or zombies process or anything you can think :)