This question already has an answer here:
i want one header on All pages but this Code only working on localhost but not working on Hosting
<?php include_once("header.php");?>
thanks
</div>
For a more CodeIgniter orientated answer, I would suggest to move your header in to your templates/views section of CI. From there, I would do the following in your controller:
$this->load->view('header', $header); //Nav Bar
$this->load->view('clients/overview', $data); //Detailed Page View
$this->load->view('footer'); //Tidy Up with Footer etc
Similar can be achieved in CakePHP.