//Insert ads after second paragraph of single post content.

add_filter( 'the_content', 'prefix_insert_post_ads' );

function prefix_insert_post_ads( $content ) {

$ad_code = '<div>Ads code goes here</div>';

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

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 );
}