add_filter( 'the_content', 'prefix_insert_post_ads' );
function prefix_insert_post_ads( $content ) {

    // checkbox to show advertisement on post, default true
    if ( get_field('show_advertisement') ) {

        // should be where I pull the custom-post-type
        // $ad_code = '<a href="'.$link.'" target="_blank"><img src="'.get_field('upload_advertisement').'" /></a>';
        //
        $random_ad = get_posts(array(
            'numberposts' => 1,
            'post_type' => 'advertising',
            'order' => 'rand'
        ));

        if (!empty($random_ad)) {
            $random_ad = array_shift($random_ad);
            $link = addhttp( get_field('advertisement_link', $random_ad->ID));
            $image = get_field('upload_advertisement', $random_ad->ID);
        }

        if ( is_single() && ! is_admin() ) {

            // field on single.php post for how many paragraphs to post the ad after
            $show_after = get_field('advertisement_show_after');

            // return the $ad_code, show ad after # paragraphs, get content
            return prefix_insert_after_paragraph( $random_ad, $show_after, $content );
        }

        return $content;

    } else {

        return $content;

    }
}

// Parent Function that makes the magic happen
function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
    $closing_p = '</p>';
    $paragraphs = explode( $closing_p, $content );
    foreach ($paragraphs as $index => $paragraph) {

        if ( trim( $paragraph ) ) {
            $paragraphs[$index] .= $closing_p;
        }

        if ( $paragraph_id == $index + 1 ) {
            $paragraphs[$index] .= $insertion;
        }
    }

    return implode( '', $paragraphs );
}