Dockerized Website returns Access denied for user 'test'@'localhost' (using password: YES)

I'm trying to run my website in a docker container. when I don't need a database everything works fine but while using a database I get Access denied for user 'test'@'localhost' (using password: YES). it happens even while I use root user. Here is my docker-compose.yml:

version: '3'
services:
    php-apache-environment:
        container_name: php-apache-wave
        build:
            context: ./php
            dockerfile: Dockerfile
        depends_on:
            - db
        volumes:
            - ./php/src:/var/www/html/
        ports:
            - 8000:80
    db:
        container_name: db-wave
        image: mysql
        restart: always
        volumes:
        - ./db:/docker-entrypoint-initdb.d       
        environment:
            MYSQL_ROOT_PASSWORD: test
            MYSQL_DATABASE: test
            MYSQL_USER: test
            MYSQL_PASSWORD: test
        ports:
            - "9906:3306"

and here is how I connect to db in my php file:

$conn = new mysqli("db-wave","test","test","test");

What is the problem?! everything works fine while I use xampp or other web servers. But in docker, the error persists.