Prova nedanstående kod; detta kommer att fungera
public function insert($data) {
// Inserting into your table
// Calling model
$done = $this->db->insert('sign_up', $data);
// You can do something else here
if($done) {
?>
<script>$(document).ready(function(){$('#thankyouModal').modal('show')});</script>
<div class="modal fade" id="thankyouModal" tabindex="-1" role="dialog">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
<h4 class="modal-title" id="myModalLabel">Thank you for pre-registering!</h4>
</div>
<div class="modal-body">
<p>Thanks for getting in touch!</p>
</div>
</div>
</div>
</div>
<?php
}
}