Problem mit UPDATE in MySQL

<?php
    $db = new database();
    //getting values
    if(isset($_POST['submit'])){


        $title = mysqli_real_escape_string($db->link,$_POST['title']);
        $content = mysqli_real_escape_string($db->link,$_POST['content']);
        $cat = mysqli_real_escape_string($db->link,$_POST['category_id']);
        $author = mysqli_real_escape_string($db->link,$_POST['author']);
        $tags = mysqli_real_escape_string($db->link,$_POST['tags']);


        //simple validation and query
        if($title=='' || $author=='' || $tags==''){
        echo "Bitte alle Felder ausf&uuml;llen!";
        exit();
        }
        else {
        $query = "INSERT INTO posts (category_id,title,content,author,tags)
        values ('$cat','$title','$content,'$author','$tags')";


        $insert_post = $db->insert($query);
        }
    }
    //getting posts
    $query = "SELECT * FROM categories";
    $cat = $db->select($query);
?>
 <form role="form" method="post" action="add_post.php">
 <div class="form-group">
    <label>Title</label>
    <input type="text" name="title" class="form-control" placeholder="Enter title">
  </div>


  <div class="form-group">
    <label>Inhalt</label>
    <textarea name="content" class="form-control" placeholder="Enter text" ></textarea>
  </div>
  <div class="form-group">
    <label>Kategorie</label>
  <select class="form-control" name="cat">
    <?php while($row=$cat->fetch_array()) :?>
     <option value="<?php echo $row['id'];?>"><?php echo $row['title'];?></option>
    <?php endwhile;?>
  </select>
  </div>


  <div class="form-group">
    <label>Author</label>
    <input type="text" name="author" class="form-control" placeholder="Enter author">
  </div>


  <div class="form-group">
    <label>Tags</label>
    <input type="text" name="tags" class="form-control" placeholder="Enter tags">
  </div>
  <div>
  <button type="submit" name="submit" class="btn btn-success">Hinzuf&uuml;gen</button>
  <a href="index.php" class="btn btn-danger"/>L&ouml;schen</a>
  </div><br/>
</form>