Böyleb bir şey denedim:

<div class="right-side">
                <?php
                    if (isset($_SESSION['giris'])) { ?>
                    <form action="" method="post">
                        <input name="cikis" type="submit" value="Çıkış">
                    </form>
                <?php 
                        if ($_POST['cikis']) {
                            session_start();
                            session_unset();
                            session_destroy();
                            require "phps/yonlendirme.php";
                        }
                } 
                    else{
                ?>
                <ul class="login">
                    <li style="line-height: 60px">
                        <div class="popup-container">
                            <label class="button" for="login-popup">LOGIN</label>
                            <input type="checkbox" id="login-popup">
                            <div class="popup">
                                <label for="login-popup"></label>
                                <div class="inner">
                                    <div class="title">
                                        <h6>LOGIN</h6>
                                        <label for="login-popup">
                                            <i class="fa fa-times"></i>
                                        </label>
                                    </div>
                                    <div class="content">
                                        <form method="get">
                                            <ul>
                                                <li>
                                                    <input type="email" name="gemail" placeholder="E-mail address">
                                                </li>
                                                <li>
                                                    <input type="password" name="gpass" placeholder="Password">
                                                </li>
                                                <li>
                                                    <input name="giris" class="giriskayit" type="submit" value="Kayıt Ol!">
                                                </li>
                                            </ul>
                                        </form>
                                        <?php
                                            if($_GET){
                                                session_start();
                                                $gemail = $_GET['kemail'];
                                                $gpass = $_GET['kpass'];
                                                if($kemail!="" && $kpass!=""){
                                                    $check=$db->prepare("SELECT * FROM hesapbilgileri WHERE eposta=? and sifre=?");
                                                    $check->execute([$gemail, $gpass]);
                                                    $checksayi=$check->rowcount();
                                                    if ($checksayi > 0) {
                                                        $_SESSION["giris"] = $gemail;
                                                        echo "Giriş başarılır...";
                                                        require_once "yonlendirme.php";
                                                    }
                                                }
                                            }
                                        ?>
                                    </div>
                                </div>
                            </div>
                        </div>
                        <div class="popup-container">
                            <label class="button" for="register-popup">REGISTER</label>
                            <input type="checkbox" id="register-popup">
                            <div class="popup">
                                <label for="register-popup"></label>
                                <div class="inner">
                                    <div class="title">
                                        <h6>REGISTER</h6>
                                        <label for="register-popup">
                                            <i class="fa fa-times"></i>
                                        </label>
                                    </div>
                                    <div class="content">
                                        <form method="post">
                                            <ul>
                                                <li>
                                                    <input type="text" name="username" placeholder="Username">
                                                </li>
                                                <li>
                                                    <input type="email" name="kemail" placeholder="E-mail address">
                                                </li>
                                                <li>
                                                    <input type="password" name="kpass" placeholder="Password">
                                                </li>
                                                <li>
                                                    <input name="kayit" class="giriskayit" type="submit" value="gönder">
                                                </li>
                                            </ul>
                                        </form>
                                        <?php
                                            if($_POST){
                                                $username = $_POST['username'];
                                                $kemail = $_POST['kemail'];
                                                $kpass = $_POST['kpass'];
                                                if(!$username || !$kemail || !$kpass){
                                                    echo "Boş yer bırakmayınız...";
                                                }
                                                else{
                                                    $yukle = $db->prepare("INSERT  INTO hesapbilgileri set username=?, eposta=?, sifre=?");
                                                    $yukle->execute(array($username, $kemail, $kpass));
                                                    if($yukle){
                                                        echo "Başarılı bir şekilde yüklendi...";
                                                        require_once "yonlendirme.php";
                                                    }
                                                    else{
                                                        echo "Başamadılı olamadı...";
                                                    }
                                                }
                                            }
                                        ?>
                                    </div>
                                </div>
                            </div>
                        </div>
                    </li>
                    <?php 
                        try {
                            $db = new PDO("mysql:host=localhost;dbname=proje;charset=utf8", "root","");
                        } catch ( PDOException $e ){
                            print $e->getMessage();
                        }
                        $sorgu=$db->query("SELECT * FROM hesapbilgileri")->fetch(PDO::FETCH_ASSOC);
                            if($sorgu['admin'] == 1) { ?>
                               <li style="padding-right: 1rem">
                                    <a href="admin.php">Admin paneli</a>
                                </li>
                    <?php } ?>
                </ul>
                 <?php } ?>
            </div>

Ama "Çıkış" kısmı giris session' ı olasına rağmen gözükmedi