$ds = array(
        0 => array("pipe", "r"),
        1 => array("pipe", "w"),
        2 => array("pipe", "w")
    );
    $proc = array();
    $pipe_fd = array();

    define('STDIN_FILENO', 0);
    define('STDOUT_FILENO', 1);
    define('STDERR_FILENO', 2);

    if (!is_resource($proc[0] = proc_open("dot", $ds, $pipe_fd[0]))) {
        die("Error creating proc[0]");
        return;
    }

    $scriptname = tempnam("/tmp", "diff");
    $script = <<<SCRIPT
N[$.nlp!=""] {
    double x, y, lx, ly;
    sscanf($.pos,"%f,%f",&x,&y);
    sscanf($.nlp,"%f,%f",&lx,&ly);
    $.xlp = sprintf("%.03f,%.03f",x+lx,y+ly);
}
SCRIPT;
    $script_fd = fopen($scriptname, "w");
    fwrite($script_fd, $script);
    fclose($script_fd);

    if (!is_resource($proc[1] = proc_open("gvpr -c -f $scriptname", $ds, $pipe_fd[1]))) {
        die("Error creating proc[1]");
        proc_close($proc[0]);
        return;
    }

    if (!is_resource($proc[2] = proc_open("neato -Tsvg -n2", $ds, $pipe_fd[2]))) {
        die("Error creating proc[2]");
        proc_close($proc[0]);
        proc_close($proc[1]);
        return;
    }

    echo "Send dot file\n";
    // Send dot file
    fwrite($pipe_fd[0][STDIN_FILENO], $dot);
    fclose($pipe_fd[0][STDIN_FILENO]);

    // Read from proc[0]
    $sc = stream_get_contents($pipe_fd[0][STDOUT_FILENO]);
    fclose($pipe_fd[0][STDOUT_FILENO]);
    fclose($pipe_fd[0][STDERR_FILENO]);

    // Write to proc[1]
    fwrite($pipe_fd[1][STDIN_FILENO], $sc);
    fclose($pipe_fd[1][STDIN_FILENO]);

    // Read from proc[1]
    $sc = stream_get_contents($pipe_fd[1][STDOUT_FILENO]);
    fclose($pipe_fd[1][STDOUT_FILENO]);
    fclose($pipe_fd[1][STDERR_FILENO]);

    // Write to proc[2]
    fwrite($pipe_fd[2][STDIN_FILENO], $sc);
    fclose($pipe_fd[2][STDIN_FILENO]);

    // Read from proc[2]
    $svg = stream_get_contents($pipe_fd[2][STDOUT_FILENO]);
    fclose($pipe_fd[2][STDOUT_FILENO]);
    fclose($pipe_fd[2][STDERR_FILENO]);

    proc_close($proc[0]);
    proc_close($proc[1]);
    proc_close($proc[2]);
    unlink($scriptname);