Theme interface segregation - proof of concept

functions.php

<?php
// ...
require_once dirname(__FILE__)."/Theme.php

ReadingTimeFeature.php

<?php
namespace PublicFunctionOrg\WordPress\Theme\Sample;
interface ReadingTimeFeature{
  public function render($post_id);
}

ReadingTimeFeatureImpl.php

<?php
namespace PublicFunctionOrg\WordPress\Theme\Sample;
class ReadingTimeFeatureImpl implements ReadingTimeFeature {
  public function render($post_id){
    if(function_exists('reading_time_render')){
      reading_time_render($post_id);
    }
  }
}

single.php

<?php
get_header();
// ... all the html page rendering
while ( have_posts() ) :
  the_post();
  // ... post html rendering
  
  \PublicFunctionOrg\WordPress\Theme\Sample\Theme::readingTime()->render(get_the_ID());

  // ... even more post html rendering
endwhile;
// ... even more html page rendering
get_sidebar();
get_footer();

Theme.php

<?php
namespace PublicFunctionOrg\WordPress\Theme\Sample;
class Theme {
  
  private function __construct(){
    // https://medium.com/write-better-wordpress-code/psr-4-autoloading-in-plugins-and-themes-720f459f4df4
    require_once dirname(__FILE__)."/vendor/autoload.php";
  }
  
  // ------------------------------
  // singleton
  // ------------------------------
  private static $instance;
  public static function instance(): self{
    if(static::$instance == null){
      static::$instance = new static();
    }
    return static::$instance;
  }
  
  // ------------------------------
  // feature access
  // ------------------------------
  public function readingTime(): ReadingTimeFeature {
    if($this->readingTimeFeature == null){
      $this->readingTimeFeature = new ReadingTimeFeatureImpl();
    }
    return $this->readingTimeFeature;
  }
}