使用tinymce [duplicate]更新文本时sql查询损坏

i am creating news letter system in php ,every thing work fine but problem occurs when i try to compose newsletter using tinymce wysiwyg editor ,it cause the corruption of sql query and says this error

You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'book antiqua', palatino, serif;">Hello every one this is my newsletter system

this is my insertation query

<?php 
include '../database/connection.php';
    $subject=$_POST['subject'];
        $message=$_POST['message'];
        $update=mysqli_query($con,"UPDATE `letters` SET `subject` = '$subject' ,`content` = '$message' WHERE `letters`.`id` = $id
     ")or die(mysqli_error($con)) ;
?>

and this is html form

<form action="" method="post">
  <div class="form-group">
   <input type="text" class="form-control" name="subject"  value="<?php echo $news_title?>">
    </div>

    <div class="form-group">
   <textarea class="form-control" id="mytextarea(tinymce id )" name="message"  ><?php echo $news_msg?></textarea>
  </div>

    <button type="submit" name="submit" class="btn btn-primary btn-block">Draft The Newsletter</button>

</form>

</div>