$exists = mysql_num_rows(mysql_query("SELECT seo FROM category_nav, category_sub, media, page WHERE category_nav.seo='".$url."' OR category_sub.seo='".$url."' OR media.seo='".$url."' OR page.seo='".$url."'"));
    if ($exists > 0) {
        echo 'Böyle bir link mevcut...';
        exit;
    }