If you use Zend Optimizer 1.20, it is not compatible with PCNTL.  You'll
get an error message stating: '[Zend Optimizer] Zend Optimizer 1.2.0 is
incompatible with pcntl 1.0 in Unknown on line 0'.

You get this error message when you compile using the --enable-pcntl
option.

This piece of code helped me to find out what signals are being sent to my
process:

function sig_identify($signo) {
  switch($signo) {
    case SIGFPE:    return 'SIGFPE';
    case SIGSTOP:   return 'SIGSTOP';
    case SIGHUP:    return 'SIGHUP';
    case SIGINT:    return 'SIGINT';
    case SIGQUIT:   return 'SIGQUIT';
    case SIGILL:    return 'SIGILL';
    case SIGTRAP:   return 'SIGTRAP';
    case SIGABRT:   return 'SIGABRT';
    case SIGIOT:    return 'SIGIOT';
    case SIGBUS:    return 'SIGBUS';
    case SIGPOLL:   return 'SIGPOLL';
    case SIGSYS:    return 'SIGSYS';
    case SIGCONT:   return 'SIGCONT';
    case SIGUSR1:   return 'SIGUSR1';
    case SIGUSR2:   return 'SIGUSR2';
    case SIGSEGV:   return 'SIGSEGV';
    case SIGPIPE:   return 'SIGPIPE';
    case SIGALRM:   return 'SIGALRM';
    case SIGTERM:   return 'SIGTERM';
    case SIGSTKFLT: return 'SIGSTKFLT';
    case SIGCHLD:   return 'SIGCHLD';
    case SIGCLD:    return 'SIGCLD';
    case SIGIO:     return 'SIGIO';
    case SIGKILL:   return 'SIGKILL';
    case SIGTSTP:   return 'SIGTSTP';
    case SIGTTIN:   return 'SIGTTIN';
    case SIGTTOU:   return 'SIGTTOU';
    case SIGURG:    return 'SIGURG';
    case SIGXCPU:   return 'SIGXCPU';
    case SIGXFSZ:   return 'SIGXFSZ';
    case SIGVTALRM: return 'SIGVTALRM';
    case SIGPROF:   return 'SIGPROF';
    case SIGWINCH:  return 'SIGWINCH';
    case SIGPWR:    return 'SIGPWR';
  }
}

function sig_handler($signo) {
  echo "Caught " . sig_identify($signo) . " (" .
$signo  . ") on " . posix_getpid() . "\n";
}

pcntl_signal(SIGFPE, "sig_handler");
pcntl_signal(SIGHUP, "sig_handler");
// pcntl_signal(SIGINT, "sig_handler");
pcntl_signal(SIGQUIT, "sig_handler");
pcntl_signal(SIGILL, "sig_handler");
pcntl_signal(SIGTRAP, "sig_handler");
pcntl_signal(SIGABRT, "sig_handler");
pcntl_signal(SIGIOT, "sig_handler");
pcntl_signal(SIGBUS, "sig_handler");
pcntl_signal(SIGPOLL, "sig_handler");
pcntl_signal(SIGSYS, "sig_handler");
pcntl_signal(SIGCONT, "sig_handler");
pcntl_signal(SIGUSR1, "sig_handler");
pcntl_signal(SIGUSR2, "sig_handler");
pcntl_signal(SIGSEGV, "sig_handler");
pcntl_signal(SIGPIPE, "sig_handler");
pcntl_signal(SIGALRM, "sig_handler");
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGSTKFLT, "sig_handler");
pcntl_signal(SIGCHLD, "sig_handler");
pcntl_signal(SIGCLD, "sig_handler");
pcntl_signal(SIGIO, "sig_handler");
pcntl_signal(SIGTSTP, "sig_handler");
pcntl_signal(SIGTTIN, "sig_handler");
pcntl_signal(SIGTTOU, "sig_handler");
pcntl_signal(SIGURG, "sig_handler");
pcntl_signal(SIGXCPU, "sig_handler");
pcntl_signal(SIGXFSZ, "sig_handler");
pcntl_signal(SIGVTALRM, "sig_handler");
pcntl_signal(SIGPROF, "sig_handler");
pcntl_signal(SIGWINCH, "sig_handler");
pcntl_signal(SIGPWR, "sig_handler");

I commented out SIGNIT, as it is the signal which is sent to your process
when you press CTRL-C. If you catch this signal, you must handle it
properly:

function sig_handler($signo) {
  switch($signo) {
    case SIGINT:
      // customized cleanup code
      exit; // now exit
      break;
  }
}

Otherwise the only possibility to stop your process is by sending a
SIGKILL signal - you can do this on the shell by typing "kill -9
PID" (where -9 is the numerical value for SIGKILL).

Note: You cannot add a handler (i.e. ignore signals) for SIGSTOP and
SIGKILL - for obvious reasons.

The signal handler pcntl_signal() does not
work if you are waiting for socket connections
with socket_accept(). The signal handler is
installed, but does not do anything if a signal
arrives. Anyone got a solution for that?

You have to use socket_select before socket_accept, so your code will wait
for connection with select. socket_select can be interrupted by signals
easily. Below is an example from my library (methods of class TNetSocket):
  //-- select
  function select($aread=NULL,$awrite=NULL,$aexcept=NULL,$timeout=NULL)
  {
    while(1)
    {
      $res="";
      $res=socket_select($aread, $awrite, $aexcept, $timeout);

      // if errno===0 it means what select was interrrupted by SysV signal
      if($res===false &&
socket_last_error($this->socket())!==0)
      { // error occured, interrupted not by a signal
        $this->set_socket_error(__LINE__);
        return(false);
      }
      break;
    }
    return(true);
  }

  //-- accept, wait for incomming connection
  function accept()
  {
    $this->clear_socket_error();
    $this->set_io_socket(_SOCKET_);

    $socket=$this->socket();
    $aread=array($socket);
    if ($this->select($a=&$aread)===false)
      return(false);

    $child_socket=socket_accept($this->socket);
    if($child_socket <= 0)
    { // error occured
      $this->set_socket_error(__LINE__);
      return(false);
    }

    $this->child_socket=$child_socket;
    $this->sockets[_CHILD_SOCKET_]=&$this->child_socket;
    $this->set_io_socket(_CHILD_SOCKET_);

    $a=&$this->peername;
    $res=socket_getpeername($child_socket,$a);

    if($res <= 0)
    { // error occured
      $this->set_socket_error(__LINE__);
      return(false);
    }

    $this->get_address_and_port(_CHILD_SOCKET_);
    TLogManager::phpserv("Connection accepted. ADDRESS
$this->address, PORT
$this->port","net_socket",__FILE__,__LINE__);

    $this->connected=true;
    return(true); // return new object of TNetSocket type
  }