He guys! Today I will show you how to modify a post in WordPress so that you can have your ads or some customized contents just after the second paragraph of your WordPress single posts. Just look at the following code snippets and you will find the explanation inside code snippets. It’s well documented for your usage. If you find it helpful, don’t forget to comment below.

//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 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 );
}