代码实现的方式很简单，只要在下面functions.php插入下面的代码即可实现（记得将：你的广告代码替换掉）

// 插入广告所需的功能代码
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( '', $paragraphs );
}
//以下是广告代码修改位置
add_filter( 'the_content', 'prefix_insert_post_ads' );
function prefix_insert_post_ads($content){
    $pattern = "/<p>.*?<\/p>/";
    $paragraph_count = preg_match_all($pattern,$content); //计算文章的段落数量
    if($paragraph_count >= 5 && is_single()){//如果文章的段落数量少于5段，则不会插入文章段落广告
        $paragraph_count -=2;
        $insert_paragraph=rand(3,$paragraph_count);
        $ad_code = '<div>广告代码位置，自己修改</div>';
        return prefix_insert_after_paragraph( $ad_code, $insert_paragraph, $content );
    }
    return $content;
}