php kodlarım hata var

<?php

    include("baglan.php");

    ob_start();

    session_start();

    $kadi = isset($_POST['ad']);

    $sifre = isset($_POST['sifre']);

    $sql_check = mysqli_query($conn, "select * from kt where email ='" . $kadi . "' and sifre='" . $sifre . "' ") or die(mysqli_error());

    if (mysqli_num_rows($sql_check)) {

        $_SESSION["login"] = "true";

        $_SESSION["user"] = $kadi;

        $_SESSION["pass"] = $sifre;

        header("Location:index.php");

        echo "1";

    } else {

        echo "2";

        if ($kadi == "" or $sifre == "") {

            echo "<center>Lutfen kullanici adi ya da sifreyi bos birakmayiniz..! <a href=javascript:history.back(-1)>Geri Don</a></center>";

            echo "3";

        } else {

            echo "4";

            echo "<center>Kullanici Adi/Sifre Yanlis.<br><a href=javascript:history.back(-1)>Geri Don</a></center>";

        }

    }

    echo ".$kadi";

    ob_end_flush();

    ?>