Explain inherited legacy code before you have to change it

Fill in

{{code}}

The code you need to understand, pasted in fulle.g. next_invoice_no() from an old PHP billing module

{{change}}

The change you've been asked to makee.g. Add credit notes with their own CN number sequence

{{level}}

Your background, so the explanation is pitched righte.g. Comfortable with PHP 8 and Laravel, never touched raw mysqli code

Example input

change: Add credit notes. They need their own number sequence with a CN prefix, separate from invoices.
level: Comfortable with PHP 8 and Laravel, never touched raw mysqli code.
code:
```php
function next_invoice_no($db, $branch) {
    $y = date('y');
    $m = date('n');
    $fy = $m < 4 ? ($y - 1) . $y : $y . ($y + 1);
    $db->query("LOCK TABLES invoice_seq WRITE");
    $r = $db->query("SELECT last_no FROM invoice_seq WHERE branch='$branch' AND fy='$fy'");
    $row = $r->fetch_assoc();
    if (!$row) {
        $n = 1;
        $db->query("INSERT INTO invoice_seq (branch, fy, last_no) VALUES ('$branch', '$fy', 1)");
    } else {
        $n = $row['last_no'] + 1;
        $db->query("UPDATE invoice_seq SET last_no=$n WHERE branch='$branch' AND fy='$fy'");
    }
    $db->query("UNLOCK TABLES");
    return strtoupper(substr($branch, 0, 3)) . '/' . $fy . '/' . str_pad($n, 5, '0', STR_PAD_LEFT);
}
```