(PECL ev >= 0.2.0)
    EvChild
    watchers trigger when the process receives a
    SIGCHLD
    in response to some child status changes (most typically when a child dies
    or exits). It is permissible to install an
    EvChild
    watcher after the child has been forked(which implies it might have
    already exited), as long as the event loop isn't entered(or is continued
    from a watcher), i.e. forking and then immediately registering a watcher
    for the child is fine, but forking and registering a watcher a few event
    loop iterations later or in the next callback invocation is not.
   
It is allowed to register EvChild watchers in the default loop only.
$pid
   
   , 
    bool
     $trace
   
   , 
    callable
     $callback
   
   [, 
    mixed
     $data
     = NULL
   
   [, 
    int
     $priority
     = 0
   
  ]] )$pid
   
   , 
    bool
     $trace
   
   , 
    callable
     $callback
   
   [, 
    mixed
     $data
   
   [, 
    int
     $priority
   
  ]] )
       Readonly
       . The process ID this watcher watches out for, or
       0
       , meaning any process ID.
      
Readonly .The process ID that detected a status change.
Readonly . The process exit status caused by rpid .