Implementation of captcha in sign up form

[eluser]Thorpe Obazee[/eluser]

Code:

if ($word == '')
   {
        $pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        $str = '';
        for ($i = 0; $i < 8; $i++)
        {
            $str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
        }
        
        $word = $str;
   }

I believe the 8 (8-1) is the word count.