PHP proc_open multiple input

Question

I want to start a CLIPS process using proc_open function in order to interact with a CLIPS script. So if i set it like this:

$descriptorspec = array(
    0 => array("pipe", "r"),
    1 => array("pipe", "w"),
    2 => array('pipe', 'r') // 2 is STDERR for process
);
$process = proc_open('./clips  -f troubleshooter.clp', $descriptorspec, $pipes);
if (is_resource($process)) {
    fwrite($pipes[0], '(exit)\n');
    fclose($pipes[0]);
    echo stream_get_contents($pipes[1]);
    fclose($pipes[1]);
    fclose($pipes[2]);
    proc_close($process);
}

all seems to be fine. The process is started and ended correct

But when sending multiple input to the CLIPS shell like:

if (is_resource($process)) {
    fwrite($pipes[0], '(run)\n');
    fwrite($pipes[0], '(exit)\n');
    fclose($pipes[0]);
    echo stream_get_contents($pipes[1]);
    fclose($pipes[1]);
    fclose($pipes[2]);
    proc_close($process);
}

then the process is not terminated and the PHP hangs at proc_close and only when killing the process from the system monitor it finishes. Checking the stream_get_contents output seems that the (exit) input is never sent to the process. Also when calling fclose($pipes[0]) the CPU usage of the CLIPS process from 0% gets straight to 100%. What could be the reason for this?