Using PostProcessBuilder (copy/mirror file)

// [paste in build.yaml]
// post_process_builders:
//   copy_php_file_builder:
//     target: "provides_builder"
//     import: "package:provides_builder/builders.dart"
//     builder_factory: "copyPhpFileBuilder"
//     input_extensions: [".php"]

import 'dart:async';

import 'package:build/build.dart';
import 'package:build_runner/build_runner.dart';

class _CopyPhpFileBuilder extends PostProcessBuilder {
  @override
  final inputExtensions = const ['.php'];

  final String defaultContent;

  _CopyPhpFileBuilder.fromOptions(BuilderOptions options)
      : defaultContent = options.config['default_content'] as String;

  @override
  Future<Null> build(PostProcessBuildStep buildStep) async {}
}

PostProcessBuilder copyPhpFileBuilder(BuilderOptions options) =>
    new _CopyPhpFileBuilder.fromOptions(options);