Bilder mit Beschreibung POSTEN = Session verschwindet

<section class="sec_index_two">
    <form action="" method="post" enctype="multipart/form-data">
    <input type="file" name="pic" id="file" class="inputfile"/>
    <label for="file"> Upload</label>
    </section>
    
    <section>
    <textarea name="post" placeholder="Poste etwas..."></textarea>
    </section>
    
    <section class="sec_index_three">
    <input type="hidden" name="uid"></input>
    <input type="submit" name="send_post" value="Posten"></input>
    <?php
    if(isset($_POST['send_post'])) {
        
        $_SESSION['id'] = $_POST['uid'];
        $uid = $_SESSION['id'];
        $post = $_POST['post'];
        
        $filename = $_FILES['pic']['name'];
        $size = $_FILES['pic']['size'];
        $endung = strtolower(pathinfo($_FILES['pic']['name'], PATHINFO_EXTENSION));
        $img = file_get_contents($_FILES['pic']['tmp_name']);
        
        if($insert = $pdo->prepare("INSERT INTO u_post (post, u_id, image_post, img_type) VALUES (?, ?, ?, ?)")) {
            $insert->BindValue(1, $post);
            $insert->BindValue(2, $uid);
            $insert->BindValue(3, $img);
            $insert->BindValue(4, $endung);
            
            if(!$insert->execute()) {
                print_r($insert->errorInfo());
            } else {
                echo 'erfolgreich';
            }
        }
    }
        
    
    ?>
    </form>
    </section>
    
    <section class="sec_index_four">
    <a href="#"><i class="fa fa-globe" aria-hidden="true"></i> <span>&Ouml;ffentlich</span> <i class="fa fa-caret-down" aria-hidden="true"></i> </a>
    </section>
    </article>
    
    <article id="user_post_therad">
    <?php
    if($stmt = $pdo->prepare("SELECT * FROM user JOIN u_post ON user.id = u_post.u_id ORDER BY date DESC")) {
        if(!$stmt->execute()) {
            print_r($stmt->errorInfo());
        }
        while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    ?>
    <section class="user_post_top_left">
        <ul>
        <li class="imgbox"><?php echo '<img src="data:image/' . $row['im_type'] . ';base64,' . base64_encode($row['image']) . '"/>'; ?></li>
        <li><h2><?php echo $row['username']; ?></h2></li>
        </ul>
    </section>
        
        <section class="user_post_time">
        <p><time><small><?php echo $row['date']; ?></small></time></p>
        </section>
    </section>
    
    <section class="user_post_index">
    <p><?php echo $row['post']; ?></p>
    <p><?php echo '<img src="data:image/' . $row['img_type'] . ';base64,' . base64_encode($row['image_post']) . '"/>';  ?></p>
    <span class="user_set_value_right">
    <button type="button" class="like_set"><i class="fa fa-thumbs-o-up" aria-hidden="true"></i></button><small><a href="#">271</a></small> <button type="button" class="heart_set"><i class="fa fa-heart-o" aria-hidden="true"></i></button><small><a href="#">48</a></small>
    </span>
    
    <span class="user_set_value_left">
    <button type="button" class="comment_set"><i class="fa fa-comment-o" aria-hidden="true"></i></button> <a href="#">24.200 Kommentare</a>
    </span>
    </section>
    <?php
        }
    }
    ?>