Cannot connect via PHP to docker mysql

Question

I got a docker-compose setup with two containers: One is the php/apache service and the other container is the database (mysql).

Here is my docker-compose.yml

version: '2'
services:
   app:
     depends_on:
       - db
     links:
       - db:mysql
     build: .
     image: app
     ports:
       - "80:80"
     restart: always
     links:
       - db:db
     volumes:
       - ../:/var/www/html/

   db:
     image: mysql:latest
     restart: unless-stopped
     volumes:
       - ./db_data:/var/lib/mysql
       - ./databaseDumps:/tmp/databaseDumps
     environment:
       MYSQL_USER: "myApp"
       MYSQL_PASSWORD: "root"
       MYSQL_ROOT_PASSWORD: "root"
       MYSQL_DATABASE: "myAppDatabase"
       MYSQL_ROOT_HOST: "%"

And here is my app Dockerfile:

FROM php:7-apache

COPY prefilled_files/000-default.conf /etc/apache2/sites-available/000-default.conf
RUN apt-get -qq update
RUN apt-get -qq -y install libpng-dev curl git nano vim zip unzip mysql-client libmysqlclient-dev
RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
RUN curl -sL https://deb.nodesource.com/setup_6.x | bash -
RUN apt-get install -y nodejs
RUN npm install -g bower
RUN npm install -g gulp
RUN docker-php-ext-install pdo pdo_mysql gd mysqli
EXPOSE 8080 80

The main problem is, the mysql database and the app container working well, and I can connect to the mysql database from the app container via

$root@app: mysql -h db -u myApp -p

BUT

if I try to execute composer install on my symfony project, following error message appears:

 [Doctrine\DBAL\Driver\PDOException]        
  SQLSTATE[HY000] [2002] Connection refused  



  [PDOException]                             
  SQLSTATE[HY000] [2002] Connection refused

Here are my parameters of my app:

parameters:
    database_driver: pdo_mysql
    database_host: db
    database_port: 3306
    database_name: myAppDatabase
    database_user: myApp
    database_password: root

Why is this happening? I've read through several forums and sites but nothing helped. I tried the following solutions and nothing helped:

clearing symfony cache ;)
expose 3306 on mysql container
link app and mysql container together
removing all images and containers from my computer and reinstalled everything
I tried on windows and on ubuntu 17.04. Same behavior

connecting to a docker-compose mysql container denies access but docker running same image does not

docker-compose wordpress mysql connection refused

UPDATE:

I tried to access my database with a little php script from https://gist.github.com/chales/11359952 . PHP/My Script can actually connect to the database, so the problem has to be in with my composer install or in doctrine or in my configuration of symfony.

tl;dr

2 Docker container via docker compose
I can access the database via mysql command on the app container but not over composer install. Why?

Cannot connect via PHP to docker mysql

1 Answers