You will simply need to add package column on the right side of the operator = to add the $amount to itself.
$sql = "UPDATE `table_projects`
SET `package1`= `package1` + " . (int)$amount . "
WHERE `id` = '$userid'";
Notes:
- I have added explicit typecasting to
(int) on $amount. If it is going to be decimal/float value, you can use (float) instead.
- Your code is open to SQL injection related attacks. Even
real_escape_string cannot secure it completely. Please learn to use Prepared Statements
- If you are not going to use Prepared Statements, and if the
id ($user_id) is integer value.
I would do typecasting on it as well
$sql = "UPDATE `table_projects`
SET `package1`= `package1` + " . (int)$amount . "
WHERE `id` = '" . (int)$userid . "'";