[Docker] Uncaught Error: Call to undefined function mysql_connect()

I have setup the wordpress using docker-compose. It originally had 2 services,
– mariadb and – wordpress.

Later, i have added the 3rd service, SSL certificate. Now the yml file looks like following.

services:
    #Database
    mariadb:
        image: mariadb:10.4
        volumes:
            - ./database:/var/lib/mysql
        restart: always
        environment:
            MYSQL_ROOT_PASSWORD: password
            MYSQL_DATABASE: shoppee
            MYSQL_USER: user
            MYSQL_PASSWORD: user_password
    #ssl
    ssl:
      build:
        context: ./apache2/
        dockerfile: Dockerfile
      ports:
         - '80:80'
         - '443:443'
      volumes:
        - ./:/var/www/html
        - /home/ubuntu/certs:/var/certs
        - /home/ubuntu/configs/000-default.conf:/etc/apache2/sites-enabled/000-default.conf
        - /home/ubuntu/configs/apache2.conf:/etc/apache2/apache2.conf
    #PHP
    php:
      build:
        context: .
        dockerfile: php.dockerfile
      volumes:
        - ./:/var/www/html
volumes:
    mariadb:

when i start the services, I’m getting following error,

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /var/www/html/wp-includes/wp-db.php:1688 Stack trace: #0 /var/www/html/wp-includes/wp-db.php(632): wpdb->db_connect() #1 /var/www/html/wp-includes/load.php(545): wpdb->__construct('username_here', 'password_here', 'database_name_h...', 'mysql') #2 /var/www/html/wp-settings.php(124): require_wp_db() #3 /var/www/html/wp-config.php(120): require_once('/var/www/html/w...') #4 /var/www/html/wp-load.php(37): require_once('/var/www/html/w...') #5 /var/www/html/wp-blog-header.php(13): require_once('/var/www/html/w...') #6 /var/www/html/index.php(17): require('/var/www/html/w...') #7 {main} thrown in /var/www/html/wp-includes/wp-db.php on line 1688

Could please anyone advice on this ? I’m desperately stuck on this one.