将（sum）变量添加到数据库中的现有值

I have database with column "kredit". I want to add $dobitak to kredit.

So if is in database kredit=10, and $dobitak=15 kredit+dobitak=25 but my code return me kredit=15 in database.

$sql = "UPDATE user
    SET  kredit='kredit' + '".$dobitak."'
    WHERE id='" . $info['user_id'] . "'";

What I need to change to get correct result in database?

try this ...

$sql = "UPDATE user
    SET  kredit=kredit + ".$dobitak."
    WHERE id='" . $info['user_id'] . "'";

Because for integer type field you need not enclose with "'"

kredit = kredit + '". $dobitak ."'
WHERE id='" . $info['user_id'] . "'";

Remove the ' ' from around kredit