Docker 无法使用 PHP 连接到 mariadb

This is my docker-compose.yml file

version: "3.1"
services:

  nginx:
    image: nginx:alpine
    container_name: nginx
    volumes:
      - ./config/nginx/nginx.conf:/etc/nginx/conf.d/default.conf
    ports:
      - "80:80"
    links:
       - php

  php:
    image: php:7.1-fpm
    container_name: php
    links:
      - mariadb:mysql
    volumes:
      - ./public:/public
    ports:
      - "9000:9000"

  mariadb:
    image: mariadb:10.1
    container_name: database
    environment:
      MYSQL_ROOT_PASSWORD: admin
    ports:
      - "3306:3306"

  phpmyadmin:
    image: phpmyadmin/phpmyadmin
    container_name: phpmyadmin
    links:
      - mariadb
    ports:
      - 8183:80
    environment:
      PMA_HOST: mariadb
      PMA_USER: root
      PMA_PASSWORD: admin
      PMA_ARBITRARY: 1

我的 nginx.conf 文件:

server {
    listen 80 default;

    client_max_body_size 108M;

    access_log /var/log/nginx/application.access.log;


    root /public;
    index index.php;

    if (!-e $request_filename) {
        rewrite ^.*$ /index.php last;
    }

    location ~ .php$ {
        fastcgi_pass php:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PHP_VALUE "error_log=/var/log/nginx/application_php_errors.log";
        fastcgi_buffers 16 16k;
        fastcgi_buffer_size 32k;
        include fastcgi_params;
    }

}

<?php

    $servername = "localhost";
    $username = "root";
    $password = "admin";
    $database = "mysql";

    try {
        $conn = new PDO("sqlite:host=".$servername.";dbname=" . $database, $username, $password);
        $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

        $sql = $conn->prepare("SELECT * FROM testDB");
        $sql->execute();

        while($result = $sql->fetch(PDO::FETCH_ASSOC)){
            $result['myTestData'];
        }

    }
    catch(PDOException $e) {
        echo "Connection failed: " . $e->getMessage();
    }

?>