Insert AdSense Ad code after certain paragraph number. Please note that the following code will consider all `p` tags into its consideration while executing

wp-insert-ad-code-legacy.php

<?php
/*
Plugin Name: Insert AdSense Ad Code after certain paragraph
Plugin URI:  https://www.isaumya.com/
Description: Insert AdSense Ad code after certain paragraph number considering all `p` tags
Version:     1.0
Author:      Saumya Majumder
Author URI:  https://www.isaumya.com/
*/

//* Ad below the 1st paragraph of post
add_filter( 'the_content', function( $content ) {
	$paragraphNumber = 1;
	$ad_code = '<script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
		<!-- Responsive Ad Code -->
		<ins class="adsbygoogle center top-ad"
		style="display:block"
		data-ad-client="ca-pub-XXXXXXXXXXXX"
		data-ad-slot="XXXXXXXXXXXXX"></ins>
		<script>
			(adsbygoogle = window.adsbygoogle || []).push({});
		</script>';

	//Check if this is an AMP page then push the AMP page specific ad code
	if ( function_exists( 'is_amp_endpoint' ) ) {

		if( is_singular( 'post' ) && is_amp_endpoint() ) {
			$ad_code = '<!-- AMP Ad Code -->
			<div class="adsbygoogle">
				<amp-ad width=300 height=250
				type="adsense"
				data-ad-client="ca-pub-XXXXXXXXXXXX"
				data-ad-slot="XXXXXXXXXXXXX"></amp-ad>
			</div>';

			return ad_insert_after_paragraph( $ad_code, $paragraphNumber, $content );
		}
	}

	if ( is_singular( 'post' ) ) {
		return ad_insert_after_paragraph( $ad_code, $paragraphNumber, $content );
	}

	return $content;
}, 19);

// Parent Function that makes the magic happen
function ad_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 );
}