Read the last data and generate alphanumeric employee number - php

Question

I have a column in my database employee number and data which is ACZ2020000. When I add an employee, the generated employee number is ACZ2020002. How can I generate the ACZ2020001? Help me, please. Thanks so much, guys.
This what I did:
public function generate_employee_id() {
$query = $this->db->query("SELECT COUNT(*) AS num_rows FROM tbl_accounts");
$row = $query->fetch_object();
$number = $row->num_rows;
++$number;
$len = strlen($number);
for($i = $len; $i < 3; ++$i) {
$number = '0' . $number;
}
$employee_number = 'ACZ'. date("Y") . $number;
echo json_encode(['employee_number' => $employee_number]);
}

Answer 1

You want the highest number, something that doesn't have anything to do with the number of rows. If I understood the rules, you can do this to get the numbers already used:
SELECT employee_id, CAST(SUBSTR(employee_id, 5) AS signed) -- MySQL flavour
FROM tbl_accounts
WHERE employee_id LIKE 'ACZ20%'
... and good old MAX() to grab the latest:
SELECT MAX(CAST(SUBSTR(employee_id, 5) AS SIGNED)) + 1 AS next
FROM tbl_accounts
WHERE employee_id LIKE 'ACZ20%'
Said that, beware of potential issues caused by concurrent accesses!

Answer 2

Perhaps the best answer here would be to recommend not trying to manage your employee number sequence from your PHP application. Instead, let your database do that heavy lifting by way of an auto increment column or something similar. Note that if all employee numbers be prefaced by ACZ, then you really only need a way to generate a sequence, and this is something that databases are reasonably good at doing.