【webpack】webpack-dev-serverでphpを監視したい

前提・実現したいこと

表題の通り、webpack-dev-serverでphpファイルを監視したいのですが、devServerのopenPageでindex.phpを指定すると、phpファイルがダウンロードされてしまい、参照することが出来ません。
htmlにすると、問題なく参照されます。

phpを参照させる場合、何かほかに記述をしなければいけないとかがあるのでしょうか。

webpack.config.js

const webpack = require('webpack');
const path = require('path');
const ExtractTextWebpackPlugin = require('extract-text-webpack-plugin');
const autoprefixer = require('autoprefixer');
module.exports = {
  mode: 'development',
  entry: {
    './public/assets/application.js': './src/js/index.js',
    './public/assets/style.css': './src/sass/index.scss'
  },
  output: {
    path: __dirname,
    filename: '[name]'
  },
  devServer: {
    open: true,
    openPage: 'index.php',
    contentBase: path.join(__dirname, 'public'),
    watchContentBase: true,
    port: 3000,
  },
  devtool: "source-map",
  module: {
    rules: [{
      test: /.js$/,
      use: [{
        loader: 'babel-loader',
        options: {
          presets: 'env'
        }
      }],
      exclude: /node_modules/,
    }, {
      test: /.scss$/,
      use: [
        ExtractTextWebpackPlugin.loader, {
          loader: 'css-loader',
          options: {
            url: false,
            sourceMap: true,
            minimize: true
          }
        }, {
          loader: 'sass-loader',
          options: {
            sourceMap: true
          }
        }, {
          loader: 'postcss-loader',
          options: {
            sourceMap: true,
            plugins: [autoprefixer({
              browsers: 'last 2 versions',
              grid: true
            })]
          }
        }
      ]
    }]
  },
  plugins: [new ExtractTextWebpackPlugin('[name]'), ]
}

package.json

{
  "name": "new-webpack",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "webpack-dev-server"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "autoprefixer": "^9.3.1",
    "babel-core": "^6.26.3",
    "babel-loader": "^7.1.5",
    "babel-preset-env": "^1.7.0",
    "css-loader": "^0.28.10",
    "extract-text-webpack-plugin": "^4.0.0-beta.0",
    "node-sass": "^4.10.0",
    "postcss-loader": "^3.0.0",
    "sass-loader": "^7.1.0",
    "webpack": "^4.26.0",
    "webpack-cli": "^3.1.2",
    "webpack-dev-server": "^3.1.10"
  },
  "dependencies": {}
}