php - How to generate unique Voucher code in laravel 5.2? -

i want save unique voucher code , mix of characters , numerics , should 6 in length. using laravel framework 5.2

enter code here $data = $request->all();         unset($data['_token']);         //echo "<pre>"; print_r($data); die;         ($i=1; $i <=$data['countvoucher']; $i++) {              $voucher = new voucher;             $voucher->code = "123456";// should dynamic , unique             $voucher->percentage = $data['percentage'];             $voucher->usage  = $data['usage'];             $voucher->expirydate = $data['expirydate'];             $voucher->save();         } 

$voucher->code want save in filed can me

i using function may use bellow

 $voucher->code = $this->generaterandomstring(6);// should dynamic , unique   public  function generaterandomstring($length = 20) {         $characters = '0123456789abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz';         $characterslength = strlen($characters);         $randomstring = '';         ($i = 0; $i < $length; $i++) {             $randomstring .= $characters[rand(0, $characterslength - 1)];         }         return $randomstring;     }