If you are running PHP as CLI and as a "daemon" (i.e. in a loop), this function must be called in each loop  to check if new signals are waiting dispatching.

Note that calling pcntl_signal_dispatch() from within a signal handler invoked by a previous pcntl_signal_dispatch() will not trigger handlers for any new pending signals. This means that if you write a CLI daemon which forks off child processes in response to a signal, then those child processes won't be able to respond to signals. This gave me a headache for a while as pcntl_signal_dispatch() doesn't raise any errors when this occurs. One solution is to set a flag within the signal handler and react upon it (by forking off the needed child processes) elsewhere in the parent process's main loop.

As noted by "me at subsonic dot net" calling pcntl_signal_dispatch() from within a signal handler invoked by a previous pcntl_signal_dispatch() will not trigger handlers for any new pending signals. This seems to be true even if you pcntl_exec() a new PHP processor to execute an entirely different script.

The solution seems to be to explicitly call pcntl_signal_dispatch()inside a ticks_handler() . And use sig_handler(int) as a push function to a queue. Immediately following the call to dispatch in the ticks_handler, pop your queue doing what you would have done in the signal_handler until the queue is empty.

Well I misspoke before. It is not enough to process signals outside of the signal handler. They must be processed outside of the tick handler (explicit or implied). So... 

Register a tick handler that calls pcntl_signal_dispatch();
In the signal handler, enqueue your signal;
In the main loop of your script, process your signals;

<?php
declare(ticks=1);
global $sig_queue;
global $use_queue;
$sig_queue = array();
$use_queue = true;   // set to false to do it the old way

function tick_handler()
{
     pcntl_signal_dispatch();
}

function sig_handler($sig)
{
     global $sig_queue;
     global $use_queue;

     if(isset($use_queue) && $use_queue)
     {
          $sig_queue[] = $sig;
     }
     else
     {
          sig_helper($sig);
     }
}

function sig_helper($sig)
{
     switch($sig)
     {
     case SIGHUP:
          $pid = pcntl_fork();
          if($pid) print("forked $pid\n");
          break;

     default:
          print("unhandled sig: $sig\n");
     }
}

pcntl_signal(SIGHUP,   "sig_handler");

while(true)
{
     if($use_queue) foreach($sig_queue as $idx=>$sig)
     {
           sig_helper($sig);
           unset($sig_queue[$idx]);
     }
     sleep(1);
}
?>