I can not save form data

I am not able to save my form data to the bank.

I have the following files:

functions.php

/**
 *  Cadastro de Notas
 */
function add() {
  if (!empty($_POST['nota'])) {

    $today = 
      date_create('now', new DateTimeZone('America/Sao_Paulo'));
    $nota = $_POST['nota'];
    $aluno = $_POST['nota'];
    $nota['modified'] = $nota['created'] = $today->format("Y-m-d H:i:s");

    save('notas', $nota);
    header('location: index.php');
  }
}

add.php

<?php 
  require_once('functions.php'); 
  add();
  index2();
  ?>

<?php include(HEADER_TEMPLATE); ?>

<h2>Cadastrar Nota</h2>

<form action="add.php" method="post">
  <!-- area de campos do form -->
  <hr />
  <div class="row">
    <div class="form-group col-md-7">
      <label for="name">Aluno</label>


      <select class="form-control">

      <?php if ($alunos) : ?>
      <?php foreach ($alunos as $aluno) : ?>  

      <option value="<?php echo $aluno['AlunoID']; ?>" name="nota['AlunoID']"><?php echo $aluno['AlunoNome']; ?></option>

      <?php endforeach; ?>
      <?php else : ?>

      <option>SEM RESULTADOS</option>

      <?php endif; ?>

      </select>


    </div>
      <div class="form-group col-md-7">
      <label for="name">Materia</label>
      <input type="text" class="form-control" name="nota['MateriaID']">
    </div>
    <div class="form-group col-md-7">
      <label for="name">Nota</label>
      <input type="text" class="form-control" name="nota['Nota']">
    </div>

  </div>

  <div class="row">
     <div class="form-group col-md-2">
      <label for="campo3">Data de Cadastro</label>
      <input type="text" class="form-control" name="nota['created']" disabled>
    </div>
  </div>



  <div id="actions" class="row">
    <div class="col-md-12">
      <button type="submit" class="btn btn-primary">Cadastrar</button>
      <a href="index.php" class="btn btn-default">Cancelar</a>
    </div>
  </div>
</form>

<?php include(FOOTER_TEMPLATE); ?>

What can it be?

Any help will be welcome.

Thank you for your attention.