PHP loop sum from database - php

Question

$st = $this->db->prepare("SELECT * FROM invoices WHERE group_id=?");
$st->execute(array($id));
if($st->rowCount() >= 1){
foreach ($st as $row) {
$counter = $row["paymentAmount"];
$start = 1;
for($start; $start < $st->rowCount(); $start++) {
$counter = $counter + $row["paymentAmount"];
}
}
It actually print out $row["paymentAmount"] + $row["paymentAmount"] and so on, depending on how many $row["paymentAmount"] there is. But the problem is that the last output from $row["paymentAmount"] is 2500.
There is:
10000
10000
2500
And the result is: 7500
I want it to be: 22500
And if the last result is 3000 it shall be 23000. So what I simply need is this code to take every row from the database, just not the latest one.
Edit: I want it outside of the SQL query

Answer 1

You don't need PHP logic for something like this. The functionality is built right into SQL.
SELECT SUM(paymentAmount) FROM invoices WHERE group_id=?

Answer 2

You should let your database handle the sum unless you have a legitimate reason why it needs to be handled in PHP. The database is more efficient with this type of operation and you avoid a loop in PHP.
SELECT SUM(paymentAmount) AS TotalPaymentAmount FROM invoices WHERE group_id = ?
You can then change your PHP to return just one row:
$row = $st->fetch();
echo $row["TotalPaymentAmount"];
If you need to do this calculation outside of SQL, just change your loop:
if($st->rowCount() >= 1){
//init the counter to 0 before you loop through your rows
$counter = 0;
//the foreach will iterate over your result set and add the paymentAmount to $counter.
foreach ($st as $row) {
$counter += $row["paymentAmount"];
}
//echo results outside of the loop
echo $counter;
}

Answer 3

If you need to code this outside SQL on purpose (e.g. because you need to do further processing for each row), then I'd code this as follows:
if ($st->rowCount() >= 1) {
$counter = 0;
foreach ($st as $row) {
$counter += $row["paymentAmount"];
}
}