Please read our [styleguide](http://ellislab.com/codeigniter/user-guide/general/styleguide.html), this and line 83 need corrections to comply with it. The next one should also be `elseif` and using the default length of 8 is probably safer than 4 in the first case:

    if ($captcha_word_length < 4)
    {
        $captcha_word_length = 8;
    }
    elseif ($captcha_word_length > 15)
    {
        $captcha_word_length = 15;
    }