Insert Google ad after specific top-level text block in WordPress post

insert_ad_after_block.php

/*
 * This code is for WordPress and should be used in functions.php of the active theme.
 *
 * Before using it, replace "ADVERTISER ID" with your own advertiser ID and "SLOT ID" with your own slot ID
 * (you can get both in Google AdSense). For best results, use a reponsive ad unit.
 *
 * If your posts contain "pre", "noscript" or some other type of HTML block, add them to $block_tags.
 *
 * You can change the insertion point by changing $after_block.
 *
 * In English, there are 4.5 letters per word and 150 words per paragraph on average, which is 2025 for 3 paragraphs,
 * so a post with 2000 is short and the ad will go to the end of it.
 *
 * Suggested styling for "ad-conttent-after-p3": 300px width, floated left, on wide displays, and full width, not floated,
 * on narrow displays. The ad serving code will deliver ads that fit into the space provided on every device.
 */

add_filter( 'the_content', 'insert_ad', 999999 );
function insert_ad( $content ) {

	if ( ! is_single() ) {
		return $content;
	}

	$google_ad = '
<p class="ad-content-after-p3">
	<ins class="adsbygoogle"
		 style="display:block"
		 data-ad-client="ADVERTISER ID"
		 data-ad-slot="SLOT ID"
		 data-ad-format="auto">
	</ins>
	<script>
		(adsbygoogle = window.adsbygoogle || []).push({});
	</script>
</p>
';

	$block_tags = array( 'p' => 1, 'blockquote' => 10, 'table' => 5, 'ul' => 2, 'ol' => 2, 'div' => 3, 'dl' => 2, 'h2' => 2, 'h3' => 2, 'h4' => 2 );
	$after_block = 3;
	$start = 0;
	for ( $block = 0; $block < $after_block; $block++ ) {
		$start = stripos( $content, '<', $start ); // Find the start of the next block
		$closing_tag = '</p>'; // Fallback
		foreach ( $block_tags as $tag => $length ) {
			if ( strtolower( substr( $content, $start+1, $length ) ) == $tag ) {
				$closing_tag = "</$tag>";
				break;
			}
		}
		$end = stripos( $content, $closing_tag, $start );
		if ( $end !== false ) {
			$end += strlen( $closing_tag );
			$start = $end + 1;
		}
	}

	if ( $start < 2000 ) { // Nothing found or content too short - show ad after content
		return $content . $google_ad;
	}

	// Insert ad after 3rd block
	return substr( $content, 0, $start-1 ) . $google_ad . substr( $content, $start );
}