Uncaught TypeError: $(...).modal is not a function
    at HTMLDocument.<anonymous>

<?php
if (isset($_GET['success'])) {
  $success = base64_decode($_GET['success']);
?>
  <!-- Modal -->
<div class="modal fade" id="modalsucces" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Exitó!</h5>      
      </div>
      <div class="modal-body">
<?php 
       echo $success;
        ?>       
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-success" data-dismiss="modal">Cerrar</button>        
      </div>
    </div>
  </div>
</div>
<script>
  $( window ).ready(function() {
    $('#modalsucces').modal('show');
});
</script>
<?php
}
?>