How to Ensure minimum one Check box been Checked from a group of check box

Dear Friends,

I am validaitng form in Codeigniter, I have 5 check box with 5 different name.

**** How can i ensure that at least one check box is checked from this 5 checkbox. **** If not Checked, how can i set the error message at

set_message(' ',' ')

Check Boxs

$data['M01'] = $this->input->post('M01');
            $data['M02'] = $this->input->post('M02');
            $data['M03'] = $this->input->post('M03');
            $data['M04'] = $this->input->post('M04');
            $data['M05'] = $this->input->post('M05');

Highly expecting your support….!!!, Thanking you in advance.