Increment by one value is not work while using large length of digit?

Currently i am tried to increment value by one but is not working while taking large length of digit.

For example..

Right now st_id is store in database like G4KZ00000001 now what i want when it's called second time then value will be increment by 1 so it would be then G4KZ00000002, G4KZ00000003 etc..

Following is my code..

Value get from DB..

$lastStconId =$last_api_record['st_consignment_id'];

Then Use following condition

if(empty($lastStconId)) {

$consignment_no = 'G4KZ00000001';

}else
{

$dataid = $last_api_record['st_consignment_id'];
$con =    str_replace("G4KZ", "", $dataid);


echo $consignment_no = $con+1; // dynamic
echo $consignment_no = 'G4KZ'.$consignment_no;
}

When i print $consignment_no it's always return increment value but i want it with full string G4KZ00000002.