stdin & pcntl security

[led_belly]

I am writing a php script to handle input from Apache mod_rewrite. Essentially, mod_rewrite supplies a key to stdin which php does its thing with and then returns the output on stdout. Foreseeing the need to time how long php will wait for a key to be supplied i decided to fork the process into a timed child process (code below). Speaking to someone on IRC, they suggested this was a potnetial security hole. I would like to understand this further if anyone has comments. What should I proctect against? Is forking the process really neccessary? Thanks. btw: the code is still being developed).

#!/usr/bin/php -q
<?
//ben at gelbnet dot com
//25-Sep-2002 03:53
//php.net/fopen
class CLI
{
//GLOBALS
var $return_char = "\n";
var $timeout;
var $pid;
var $cpid = 0;

//Make sure program execution doesn't time out
//set_time_limit(0);
function CLI($timeout = 5)
{
    $this->timeout = $timeout;
    $this->pid = getmypid();
}

function _setTimeout()
{
    set_time_limit(0);

    $this->cpid = pcntl_fork();
    if ($this->cpid == 0) {
        sleep($this->timeout);
        posix_kill($this->pid, SIGTERM);
        exit;
    }
}

function _clearTimeout()
{
    posix_kill($this->cpid, SIGTERM);
}

// read_data()
// gets a line of data from STDIN and returns it
function stdin()
{
    $in = fopen("php://stdin", "r");
    if (is_object($this)) $this->_setTimeout();
    $in_string = fgets($in, 255);
    if (is_object($this)) $this->_clearTimeout();
    fclose($in);
    return $in_string;
}

// write_data($outstring)
// writes data to STDOUT
function stdout($outstring)
{
    $out = fopen("php://stdout", "w");
    fwrite($out, $outstring);
    fclose($out);
}

}

$t = new CLI();
echo $t->stdin();
?>