Changing CakePHP’s default multiple select for HasAndBelongsToMany (HABTM) relationships to use multiple checkboxes used to be an arduous task. It is now a simple option as follows.
If you have a Post model that has a HABTM relationship with a Tag model, you would use the following line to display multiple checkboxes instead of the default multiple select:
<?php echo $form->input('Tag', array('multiple' => 'checkbox')); ?>
The magic is the array(‘multiple’ => ‘checkbox’).
As you probably already know, and the reason you were searching for this solution, the multiple checkboxes are much more user friendly than the multiple select.
Thank you very very much, you just saved my day !!