PHP insert data on empty spot(s)

I have the following table:

Now, what I want to do is, perhaps someone want to be in another 'class', then it's number changes according to the class. The classess look like the following;

• 1xx => Initiatie
• 2xx => Recreatie
• 3xx => Toerisme
• 4xx => Sport

where xx represents the number of the team.

So let's take a look at an example.

Team 202, wants to change to Toerisme, in Toerisme, we have the numbers 301, 304, 305 and 306. So the system must know, if someone changes it's class to 3xx, or registers as a new user in that class, that he/she must get the first number (which is 302). The second number would then be 303, but the third will be 307.

So I have to have a way of checking free spots in those numbers.

The code I'm currently using to assign numbers is the following:

public function getTeamNumber($klasse_start_number)
{

    if(self::whereRaw('LEFT(number, 1) = '.$klasse_start_number)->count() <= 0)
    {
        switch ($klasse_start_number) {
            case '1':
            return '101';
            break;
            case '2':
            return '201';
            break;
            case '3':
            return '301';
            break;
            case '4':
            return '401';
            break;
        }
    }

    $latest_number = self::whereRaw('LEFT(number, 1) = '.$klasse_start_number)->orderBy('number', 'DESC')->latest()->first()->number;
    $number = $latest_number + 1;
    return $number;
}

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