functions.php

<?php

add_filter( 'the_content', 'prefix_insert_post_ads' );

function prefix_insert_post_ads( $content ) {
	
	$ad_code = '<div id="googleads-1"></div>';

	if ( is_single() && ! is_admin() ) {
		return prefix_insert_after_paragraph( $ad_code, 2, $content );
	}
	
	return $content;
}
 
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 );
}