Docker php_network_getaddresses error

Question

I have following Docker containers running that were generated by PHPDocker:

learn-php-mysql:
  image: mysql:5.7
  container_name: learn-php-mysql
  volumes:
    - "./.data/db:/var/lib/mysql"
  restart: always
  environment:
    MYSQL_ROOT_PASSWORD: learning
    MYSQL_DATABASE: learning
    MYSQL_USER: learning
    MYSQL_PASSWORD: learning

learn-php-webserver:
  image: phpdockerio/nginx:latest
  container_name: learn-php-webserver
  volumes:
    - ./code:/code
    - ./nginx/nginx.conf:/etc/nginx/conf.d/default.conf
  ports:
   - "8080:80"
  links:
   - learn-php-php-fpm

learn-php-php-fpm:
  build: .
  dockerfile: php-fpm/Dockerfile
  container_name: learn-php-php-fpm
  volumes:
    - ./code:/code
    - ./php-fpm/php-ini-overrides.ini:/etc/php/7.1/fpm/conf.d/99-overrides.ini
  links:
    - learn-php-mysql:mysql

Everything works fine, except for when trying to connect to MySQL server via PHP code:

$mysqli = new mysqli("db", "learning", "learning", "learning");

/* check connection */
if ($mysqli->connect_errno) {
    printf("Connect failed: %s\n", $mysqli->connect_error);
    exit();
}

It will throw following error:

Connect failed: php_network_getaddresses: getaddrinfo failed: Name or service not known

Why does this happen?

.

.

You can find repository for this here. And when running docker-compose up go to http://localhost:8080/classes/serialize.php to produce same error.

Solution

When making connection via PHP (or any other), use the container's name as host, which in case is learn-php-mysql.

Thus

$mysqli = new mysqli("learn-php-mysql", "learning", "learning", "learning");

Will work.