Salt Create error !

[Chaya_]

PHP Code:

<?php

class Vbulletin extends Plugin
{
    /**
     * Runtime values
     */
    private $username;
    private $password;
    private $email;
    private $db;

    /**
     * Receive the user information
     * @param String $username
     * @param String $password
     * @param String email
     */    
    public function register($username, $password, $email)
    {
        $this->username = $username;
        $this->password = $password;
        $this->email = $email;
        
        $this->db = $this->CI->load->database($this->CI->config->item('bridge'), TRUE);

        $this->process();
    }

    /**
     * Add the account
     */
    private function process()
    {
        $salt = $this->fetch_user_salt(30);

        $password = $this->encryptPassword($salt);
        $this->db->query("INSERT INTO ".$this->CI->config->item('forum_table_prefix')."user(`usergroupid`, `username`, `password`, `email`, `joindate`, `salt`) VALUES ('2', ?, ?, ?, ?, ?)", array($this->username, $password, $this->email, time(), $salt));
    }

    /**
     * Encrypt the password with a specific algorithm
     * @return String
     */
    private function encryptPassword($salt)
    {
        return md5(md5($salt) .md5( $this->password ));            
    }

    private function fetch_user_salt($length = 30)
    {
            $salt = '';
            for ($i = 0; $i < $length; $i++)
            {
                $salt .= chr(rand(33, 126));
            }
            return $salt;
    }  
}

What is wrong in this script ?