How i said in the comment you can use array_diff for select only the value isn’t selected :

$dbarray = array('1','2','3','4');
$selectedarray = array('1','3');
$substract = array_diff($dbarray, $selectedarray);
print_r($substract); //print 2,4

I have a form with other fields, I have a list of clients in a checkbox array. To get that list I use the database table-field and populate a list.
HTML FORM

<? if($this->clientCount > 0){?>
  <? foreach($this->client as $valuec) {?>
   <input type="checkbox" name="banner_allowed1[]" value="<?=$valuec['company_id']?>" <?if($valuec['banner_allowed']==1){echo 'checked="checked"';}?> style="margin:auto; size:20px;">&nbsp;<?=$valuec['title']?></label><br>
  <?}?>
<?}?>

Now when I submit the form, I need to know which one are unselected, because the form loads with the value selected from the database. I need to update the database table field value to zero(0) for the unselected ones.
I am finding a hard time with dynamic checkboxes to do a name value pair or get values of unselected checkboxes as post doesn’t send unselected checkbox values

MY submit page in POST, only getting checked checkbox values
              if(!empty($_POST['banner_allowed1'])){
                foreach($_POST['banner_allowed1'] as $clientid1){     
                  $update['userid'] = $uid;
                  $update['banner_allowed'] = 1;
                  $this->db->update('clientlist', $update, "id=$clientid1");
                }
              } else {

       }