/bin/sh -c CMD will fork sh and then exec CMD.
/bin/sh -c exec CMD will NOT fork and only executes CMD.

Therefore, you can get rid of this hack by prefixing your command to "exec bla bla bla".

As explained in http://bugs.php.net/bug.php?id=39992, proc_terminate() leaves children of the child process running. In my application, these children often have infinite loops, so I need a sure way to kill processes created with proc_open(). When I call proc_terminate(), the /bin/sh process is killed, but the child with the infinite loop is left running. 

Until proc_terminate() gets fixed, I would not recommend using it. Instead, my solution is to:
1) call proc_get_status() to get the parent pid (ppid) of the process I want to kill. 
2) use ps to get all pids that have that ppid as their parent pid
3) use posix_kill() to send the SIGKILL (9) signal to each of those child pids
4) call proc_close() on process resource

<?php
$descriptorspec = array(
0 => array('pipe', 'r'),  // stdin is a pipe that the child will read from
1 => array('pipe', 'w'),  // stdout is a pipe that the child will write to
2 => array('pipe', 'w')   // stderr is a pipe the child will write to
);
$process = proc_open('bad_program', $descriptorspec, $pipes);
if(!is_resource($process)) {
    throw new Exception('bad_program could not be started.');
}
//pass some input to the program
fwrite($pipes[0], $lots_of_data);
//close stdin. By closing stdin, the program should exit
//after it finishes processing the input
fclose($pipes[0]);

//do some other stuff ... the process will probably still be running
//if we check on it right away

$status = proc_get_status($process);
if($status['running'] == true) { //process ran too long, kill it
    //close all pipes that are still open
    fclose($pipes[1]); //stdout
    fclose($pipes[2]); //stderr
    //get the parent pid of the process we want to kill
    $ppid = $status['pid'];
    //use ps to get all the children of this process, and kill them
    $pids = preg_split('/\s+/', `ps -o pid --no-heading --ppid $ppid`);
    foreach($pids as $pid) {
        if(is_numeric($pid)) {
            echo "Killing $pid\n";
            posix_kill($pid, 9); //9 is the SIGKILL signal
        }
    }
        
    proc_close($process);
}

?>

on Windows platform proc_terminate() does not kill sub-processes that are not handling kill signals. It happens even if you call xxx.exe and call proc_terminate() the process will remain active.

The solution is instead of calling proc_terminate() is to call the user-defined kill() function (already win/unix optimized)
After that need to close all pipes and execute proc_close().

function kill($pid){ 
    return stripos(php_uname('s'), 'win')>-1  ? exec("taskkill /F /T /PID $pid") : exec("kill -9 $pid");
}

function killall($pids) { 
    $os=stripos(php_uname('s'), 'win')>-1;
    ($_=implode($os?' /PID ':' ',$pids)) or ($_=$pids);
    return preg_match('/success|close/', $os ? exec("taskkill /F /T /PID $_") : exec("kill -9 $_"));
}

Example:

$pstatus = proc_get_status($resource);
$PID = $pstatus['pid'];

// other commands

kill($PID); // instead of proc_terminate($resource);
fclose($pipes[0]); 
fclose($pipes[1]); 
fclose($pipes[2]);
proc_close($resource);

Just a small note so people don't have to look elsewhere

To get the list of processes or find a process by name, use
$proclist = shell_exec('ps -elF') for linux
$proclist = shell_exec('tasklist') for windows

After that, you can use php normal parsing functions to get the pid