The section about Process Control Functions maybe of interest for you.
That is not part of POSIX, those are only present as you listed on linux systems - some other systems have a /proc with different things in it (sometimes stuff that's symbolic links on linux will be hardlinks, textfiles on linux will be binary, or different files with different information) or none at alldon'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 :)