Inserting ads within content

Have faith 😉

Unfortunately, your code ain’t correct in multiple ways. It assumes that the content only consists of content enclosed by paragraphs and doesn’t work correctly if not.

This would be one solution:

add_filter('the_content', 'prefix_insert_post_ads');


/**
 * Content Filter 
 */
function prefix_insert_post_ads($content) {

    $insertion = 'Your AD Code';

    if (is_single() && ! is_admin()) {
        return prefix_insert_after_paragraphs($content, $insertion, array(4,20));
    }

    return $content;

}

// Function that makes the magic happen correctly

function prefix_insert_after_paragraphs($content, $insertion, $paragraph_indexes) {

    // find all paragraph ending offsets

    preg_match_all('#</p>#i', $content, $matches, PREG_SET_ORDER+PREG_OFFSET_CAPTURE);

    // reduce matches to offset positions

    $matches = array_map(function($match) {
        return $match[0][1] + 4; // return string offset + length of </p> Tag
    }, $matches);

    // reverse sort indexes: plain text insertion just works nicely in reverse order

    rsort($paragraph_indexes); 

    // cycle through and insert on demand

    foreach ($paragraph_indexes as $paragraph_index) {
        if ($paragraph_index <= count($matches)) {
            $offset_position = $matches[$paragraph_index-1];
            $content = substr($content, 0, $offset_position) . $insertion . substr($content, $offset_position);
        }
    }

    return $content;

}

Best regards from Salzburg!