(PECL ev >= 0.2.0)
EvChild::__construct — Constructs the EvChild watcher object
$pid
   
   , 
    bool
     $trace
   
   , 
    callable
     $callback
   
   [, 
    mixed
     $data
     = NULL
   
   [, 
    int
     $priority
     = 0
   
  ]] )Constructs the EvChild watcher object.
   Call the callback when a status change for process ID
   pid
   (or any
   PID
   if
   pid
   is
   0
   ) has been received(a status change happens when the process terminates or
   is killed, or, when
   trace
   is TRUE, additionally when it is stopped or continued). In other words,
   when the process receives a
   SIGCHLD
   ,
   Ev
   will fetch the outstanding exit/wait status for all changed/zombie children
   and call the callback.
  
It is valid to install a child watcher after an EvChild has exited but before the event loop has started its next iteration. For example, first one calls fork , then the new child process might exit, and only then an EvChild watcher is installed in the parent for the new PID .
   You can access both exit/tracing status and
   pid
   by using the
   rstatus
   and
   rpid
   properties of the watcher object.
  
The number of PID watchers per PID is unlimited. All of them will be called.
The EvChild::createStopped() method doesn't start(activate) the newly created watcher.
pid
    
      Wait for status changes of process PID(or any process if PID is
      specified as
      0
      ).
     
trace
    
      If FALSE, only activate the watcher when the process terminates.
      Otherwise(TRUE) additionally activate the watcher when the process is
      stopped or continued.
     
callback
    See Watcher callbacks .
data
    Custom data associated with the watcher.
priority
    Returns EvChild object on success.