I'd try to create the form like this:
echo $this->Form->create('Post', array('type' => 'GET', 'url' => array('controller' => 'yourpostcontroller', 'action' => 'search')));
echo $this->Form->input('search', array('class' => 'form-control'));
echo $this->Form->button('Search', array('div' => false, 'class' => 'btn'));
echo $this->Form->end();
In your "Search" action (inside the controller) do not forget to read the request:
if(!empty($this->request->query['search']) ...{
//do something
}
Hope it helps.