$banana = '1,2,3,6,7';

$arr = range(1,7);// range in which you want to check
$banana = '1,2,3,6,7';
$bananaToArray = explode(',',$banana);
$res = array_diff($arr, $bananaToArray);
echo '<pre>';
print_r($res);

Array
(
 [3] => 4
 [4] => 5
)