I want to achieve
Currently, I am making a calendar application, as it is in the subject, but when you press the date I want to register the schedule, a modal window appears, write the contents there, and press the save button to send the contents to the DB I want to

Previously, I switched to another screen, typed the contents there and sent it to the DB, I thought that it would be possible to follow the procedure in the same way, but I am having trouble and I am in trouble.

I would appreciate it if you could teach me.

Source code

 try {
    if (!empty($_POST)) {
      if ($_POST['message'] !=='') {
        $content = $db->prepare('INSERT INTO posts(user_id,content) values(?,?)');
        header('Location: index.php');
  }catch(Exception $e){
    echo $e->getMessage();
    echo $e->getTraceAsString();
<!-- 2. Modal placement -->
    <!-- 3. Modal content -->
      <!-- 4. Modal header -->
        <button type="button" data-dismiss="modal">
          <span aria-hidden="true">&times;</span>
      <!-- 5. Modal body -->
      <form action="" method="post">
          <input type='text' name='title'>
        <p>Planned content<br>
          <textarea name='message' cols='70' rows='5' ></textarea><!-- This is the content!-->
      <!-- 6. Modal footer -->
        <button type="button" data-dismiss="modal">Close</button>
        <button type="button">Save</button>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>

What I tried
There is no error, and even if you press the save button, the modal window is displayed and nothing changes.

Development environment
OS: Mac
Web server: xampp