Docker cannot connect to PMA

This is my docker-compose.yml. When i try to connect to PMA using username: root and password:1234 i get this error:

#2002 - php_network_getaddresses: getaddrinfo failed: Name does not resolve — The server is not responding (or the local server's socket is not correctly configured).

    version: "3.2"
services:
  php:
    build: 
      context: .
    image: flaviuspantucu/hedacomputers:1.0.0
    networks:
      - frontend
      - backend
    environment:
      - MYSQL_HOST=container_sql
      - MYSQL_USER=root
      - MYSQL_PASSWORD=1234
      - MYSQL_DB=hedacomputers
    volumes:
      - ./hedaComputers/:/var/www/html/
    ports:
      - "30001:80"
    container_name: container_php
  mysql:
    image: mysql:5.7
    networks:
      - backend
    environment:
      - MYSQL_ROOT_PASSWORD=1234
      - MYSQL_USER=root
      - MYSQL_PASSWORD=1234
      - MYSQL_DATABASE=hedacomputers
    container_name: container_sql
  phpmyadmin:
    image: phpmyadmin/phpmyadmin:4.7
    links:
      - mysql
    networks:
      - backend
    ports:
      - "30002:80"
    environment:
      - PMA_HOST= container_sql
      - PMA_PORT= 3306
      - MYSQL_USERNAME= root
      - MYSQL_ROOT_PASSWORD= 1234
    volumes:
      - /sessions
    container_name: container_phpmyadmin
networks:
  frontend:
  backend: