三个容器docker-composition:PHP无法连接到Maria DB

My docker-compose.yml :

version: '3'
services:
    php-apache:
        build:
            context: ./php-apache
        ports:
            - 80:80
        volumes:
            - ./DocumentRoot:/var/www/html
        links:
            - 'db'
        networks:
            - default

    db:
        image: mariadb:10.1
        volumes:
            - ./db:/var/lib/mysql
            - ./dump.sql:/docker-entrypoint-initdb.d/dump.sql            

        ports: 
            - "3306:3306"            
        environment:
            TZ: "Europe/London"
            MYSQL_ALLOW_EMPTY_PASSWORD: "no"
            MYSQL_ROOT_PASSWORD: "rootpwd"
            MYSQL_USER: 'testuser'
            MYSQL_PASSWORD: 'testpassword'
            MYSQL_DATABASE: 'testdb'
        networks:
            - default

    phpmyadmin:
        image: phpmyadmin/phpmyadmin
        links: 
            - 'db'
        ports:
            - 8000:80
        environment:
            MYSQL_USER: 'testuser'
            MYSQL_PASSWORD: 'testpassword'
            MYSQL_ROOT_PASSWORD: 'rootpwd'
            PMA_HOST: db
            PMA_PORT: 3306

This is based on a tutorial I found online.

When I run it, I can see that the PHP container is up. I can see that the db and php-mydmin are up. In fact I can successfully access the db from php-myadmin using the testuser / testpassword credentials.

But when I try to access the db from the main PHP application using

$conn = mysqli_connect("localhost","testuser",'testpassword','testdb'); 

I get

Warning: mysqli_connect(): (HY000/2002): No such file or directory in /var/www/html/index.php on line 8

Is this a problem with the docker config? Can anyone see anything missing?