Inject Adverts into Posts and Pages - Inject a specified page or widget area into post or page content. https://www.damiencarbery.com/2019/09/inject-adverts-into-posts-and-pages/

inject-ad-content.php

<?php
/*
Plugin Name: Inject Adverts into Posts and Pages
Plugin URI: https://www.damiencarbery.com/2019/09/inject-adverts-into-posts-and-pages/
Description: Inject a specified page or widget area into post or page content.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/

// Add "Ad Content" page or sidebar after the specified paragraphs.
// Modified version of: http://www.wpbeginner.com/wp-tutorials/how-to-insert-ads-within-your-post-content-in-wordpress/
add_filter( 'the_content', 'dcwd_insert_page_as_adverts' );
function dcwd_insert_page_as_adverts( $content ) {
	$ad_content_page_id = 10;
	// Add advert after these paragraphs.
	$paragraph_ids = array( 2, 4, 5, 8 );
	
	$use_page_content_for_adverts = true;
	
	if ( is_singular() && is_main_query() ) {
		// Do not add the ad content to the ad content page!
		if ( get_the_ID() == $ad_content_page_id ) {
			return $content;
		}

		// Whether to use page content or widgets for adverts.
		$ad_content = '';
		if ( $use_page_content_for_adverts ) {
			$ad_content_page_raw = get_post( $ad_content_page_id );  // http://localhost/wordpress/editorskit/ad-content/
			$ad_content = '<div class="ad-content" style="border:2px dashed #777; font-size:0.8em;">'. $ad_content_page_raw->post_content .'</div>';
		}
		else {
			ob_start();
			if ( is_active_sidebar( 'ad-content' ) ) {
				dynamic_sidebar( 'ad-content' );
			}
			$ad_content = '<div class="ad-content" style="border:2px dashed #777; font-size:0.8em;">' . ob_get_clean() . '</div>';
		}

		$closing_p = '</p>';
		$paragraphs = explode( $closing_p, $content );

		foreach ($paragraphs as $index => $paragraph) {
			// If it is not an empty paragraph (after removing whitespace at start and end)
			// then append the '</p>' again - it was removed by explode() function.
			if ( trim( $paragraph ) ) {
				$paragraphs[$index] .= $closing_p;
			}

			// Add the advert if we are after one of the specified paragraphs.
			if ( in_array( $index + 1, $paragraph_ids ) ) {
				$paragraphs[$index] .= $ad_content;
			}
		}
		
		// Optionally append the page at the end of the content too.
		//$paragraphs[] = $ad_content;

		return implode( '', $paragraphs );
    }

    return $content;
}


add_action( 'widgets_init', 'dcwd_register_adverts_sidebar' );
function dcwd_register_adverts_sidebar() {
	$args = array(
		'name'          => 'Adverts',
		'id'            => 'ad-content',
		'description'   => 'Adverts to insert into pages.',
		'class'         => 'ad-content',
		'before_widget' => '<div id="%1$s" class="widget %2$s">',
		'after_widget'  => '</div>',
		'before_title'  => '<h2 class="widgettitle">',
		'after_title'   => '</h2>'
	);
	
	register_sidebar( $args );
}