Clarifying Differences Between CHAR and VARCHAR in MySQL with CodeIgniter

I'm navigating the realms of MySQL databases, and I've encountered some confusion regarding the usage of CHAR and VARCHAR. Specifically, I'm interested in understanding their differences and how to implement them effectively in a MySQL database, especially when working with CodeIgniter. Could someone help me unravel these concepts?

Firstly, let me share a snippet of my CodeIgniter model:

PHP Code:

class User_model extends CI_Model {
    public function create_user($username, $email, $password) {
        $data = array(
            'username' => $username,
            'email' => $email,
            'password' => $password
        );

        $this->db->insert('users', $data);
        return $this->db->insert_id();
    }
}

Now, considering the MySQL table structure: 


PHP Code:

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username CHAR(20),
    email VARCHAR(255),
    password CHAR(32)
); 


Question 1: Understanding CHAR vs. VARCHAR in MySQL
I've defined the username column as CHAR(20) and the email column as VARCHAR(255). What are the key differences between CHAR and VARCHAR in MySQL, and when should one be preferred over the other? I'm particularly curious about their storage characteristics and performance implications in the context of CodeIgniter applications.

Question 2: CodeIgniter Model Best Practices
In the CodeIgniter model snippet, I'm using an array to insert data into the users table. Are there any best practices or considerations specific to CodeIgniter when working with CHAR or VARCHAR data types? Any insights on how CodeIgniter handles these data types behind the scenes?

Question 3: Effective Password Storage
I discovered that I'm using CHAR(32) for the password column to hold hashed passwords. Is this the most effective approach to store hashed passwords in MySQL with CodeIgniter? Are there any other data formats or settings that would be better suited for safeguarding and managing password data?

Question 4: The Impact of Column Length on Performance
Finally, as seen in the source, does the length given in the CHAR and VARCHAR data types have a major influence on performance, particularly when working with huge datasets in a CodeIgniter application? How should one strike a balance between defining an appropriate amount of data and improving database performance?

Your expertise on these questions would be immensely valuable as I strive to make informed decisions in my database design and CodeIgniter development. Thanks a bunch for your insights!