error strtolower function in codeigniter

The follow error I get when I try to strtolower() my checked in codeigniter. this is error:

strtolower() expects parameter 1 to be string, object given

and

 Undefined property: CI_Loader::$unit_id

this is my view

<div class="checkbox">
                                    <?php foreach ($unit_list as $unit) :?>
                                    <?php $isChecked = strtolower($unit) == strtolower($this->unit_id) ? true : false; ?>
                                            <label>
                                                <input type="checkbox" checked='$isChecked' name="unit_id[]" value="<?php echo $unit->unit_id ?>"><?php echo $unit->unit_id ?>
                                            </label>""
                                    <?php endforeach?>

                                </div>

this controller

$this->data['unit_list']= $this->munit_list->get_all_unit_list2();     

  $fleet = $this->input->post('id_fleet');
                $unit_id = $this->input->post('unit_id');
                $this->unit_id = $unit_id;

                // get shift and date
                $q = $this->database_three->select("*")->from('fleet')->where('id_fleet', $fleet)->limit(1)->get()->row();
                $fleet_date  = $q->date;
                $fleet_shift = $q->shift;

                //$this->db->select('f.*, fm.*')->from('fleet_member as fm, fleet as f');
                        $custom_sql   = "fm.id_fleet = f.id_fleet AND f.date = '$fleet_date' AND f.shift = '$fleet_shift'";
                        $custom_where = "";
                $records = array();

                for ($i=0; $i < count($unit_id) ; $i++) {
                    $records[] = array(
                                        'id_fleet' => $fleet,
                                        'unit_id'  => $unit_id[$i]
                    );

can you help me how to solve this problem?