Use of callback on a stream

Hello, I am trying to make a daemon that launches shell commands via proc_open and gets the stdout of the command via a pipe. The thing is that I would like to get this stdout via a callback, instead of monitoring the pipe regularly. I tried the following code (this is a simplified version of it, but the callback never gets called, does anyone know what I forgot ? Thanks, Olivier <?php // First we proc_open a ls

//    1 - we build the descriptor array

$descriptorspec = array(

              0 => array("pipe", "r"),  // // stdin
              1 => array("pipe", "w"),  // stdout
              2 => array("pipe", "w") // stderr
              );

$pipes=array(); // We make a simple callback function function stream_callback(){ global $pipes; $stdout_child=stream_get_contents($pipes[1]); print("Callback received:\n"); print($stdout_child."\n"); } $context=stream_context_create(array()); stream_context_set_params($context,array('notification' => 'stream_callback')); $handle=proc_open("/bin/ls /tmp",$descriptorspec,$pipes,null,null,array('context' => $context)); stream_context_set_params($pipes[1],array('notification' => 'stream_callback')); while (true){ print("Waiting...\n"); sleep(5); }