if($_POST){
  $query = $db->prepare("SELECT * FROM kullanici where email=:email LIMIT 1");
        $kayit = $query->execute(array(":email" => post('email')));
        $kayit = $query->fetch(PDO::FETCH_ASSOC);

        if ($kayit) {
            echo "Email Kayıtlı";
        } else {
            İNSERT KODLARI
        }
}