Not able to connect to mysql container from php container

I am having 2 docker containers for php app and mysql. Both are working perfectly individually. I can access my php app at localhost:8000 and can connect mysql at localhost:3306 using MySQL Workbench.

But, my php app which is inside the container is not able to connect to the mysql db which is inside another container.

My docker-compose.yml file is as follows:

    version: '3'
    services:
      website:
        container_name: php-app
        image: php-app
        build:
          context: ./
        volumes:
          - php-app:/var/www/html/
        ports:
          - 8000:80
        depends_on:
          - mysql
      mysql:
        image: mysql:8.0
        container_name: mysql-server-80
        command: --default-authentication-plugin=mysql_native_password
        volumes:
          - .:/application
        restart: always
        environment:
          - MYSQL_ROOT_PASSWORD=123456
          - MYSQL_DATABASE=testdb
          - MYSQL_USER=root
          - MYSQL_PASSWORD=123456
        ports:
          - "3306:3306"
    volumes:
      jayde-ci:
      mysql:

My database configuration file inside the codeigniter based php app is as follows:

    $db['default'] = array(
        'dsn'   => '',
        'hostname' => 'mysql',
        'username' => 'root',
        'password' => '123456',
        'database' => 'test',
        'dbdriver' => 'mysqli',
        'dbprefix' => '',
        'pconnect' => FALSE,
        'db_debug' => (ENVIRONMENT !== 'production'),
        'cache_on' => FALSE,
        'cachedir' => '',
        'char_set' => 'utf8',
        'dbcollat' => 'utf8_general_ci',
        'swap_pre' => '',
        'encrypt' => FALSE,
        'compress' => FALSE,
        'stricton' => FALSE,
        'failover' => array(),
        'save_queries' => TRUE
    );