function belirli_satir_sonrasi_reklam($content) {
    $yazi_uzunlugu = 1;
    $az_karakter = 100;

    if (is_single() && strlen($content) > $yazi_uzunlugu) {
        $before_content = substr($content, 0, $az_karakter);
        $after_content = substr($content, $az_karakter);
        $after_content = explode('</p>', $after_content);
        
        $text = 'google adsense kodu';
        
        $paragraf_sayisi = count($after_content);
        
        $ekleme_sayisi = intdiv($paragraf_sayisi - 1, 3);
        
        array_splice($after_content, 1, 0, $text);
        
        for ($i = 1; $i <= $ekleme_sayisi; $i++) {
            $index = $i * 3 + $i;
            array_splice($after_content, $index, 0, $text);
        }
        
        $after_content = implode('</p>', $after_content);
        return $before_content . $after_content;
    } else {
        return $content;
    }
}

add_filter('the_content', 'belirli_satir_sonrasi_reklam');