String helper array [closed]

My problem is that I am using the random string creator shown below, I am not using 'alpha', I am using 'fourlet'. The only problem is that this in an array and that

substr($pool, mt_rand(0, strlen($pool) -1), 1);

doesn't work as a random creator for an array of data. Can anyone please help?

Below is the full code for the helper.

{
function random_string($type = 'alnum', $len = 8)
{
    switch($type)
    {
        case 'basic'    : return mt_rand();
            break;
        case 'alnum'    :
        case 'numeric'  :
        case 'nozero'   :
        case 'alpha'    :

                switch ($type)
                {
                    case 'alpha'    :   $pool = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
                        break;
                    case 'fourlet'  :   $pool = array('my', 'fun', 'zone', '12', '24', '37');
                        break;
                }

                $str = '';
                for ($i=0; $i < $len; $i++)
                {
                    $str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
                }
                return $str;
            break;
        case 'unique'   :
        case 'md5'      :

                    return md5(uniqid(mt_rand()));
            break;
        case 'encrypt'  :
        case 'sha1' :

                    $CI =& get_instance();
                    $CI->load->helper('security');

                    return do_hash(uniqid(mt_rand(), TRUE), 'sha1');
            break;
    }
}

}

So