I am building a captcha class. I need to store the generated code in a PHP session. This is my code so far:

<?php

class captcha
{
    private $rndStr;
    private $length;

    function generateCode($length = 5)
    {
        $this->length = $length;
        $this->rndStr = md5(time() . rand(1, 1000));
        $this->rndStr = substr($rndStr, 0, $this->length);
        return $rndStr;
        if(session_id() != '')
        {
            return "session active";
        } else {
            return "no session active";
        }
    }
}

?>

And using this code to check:

<?php

include('captcha.class.php');
session_start();

$obj = new captcha();
echo $obj->generateCode();

?>

But it doesn't output anything to the page, not even a PHP error. Does someone know why this is? And is there a better way I can check if I've started a session using session_start()?

Thanks.