For Loop Stops after First Iteration - mysql

Question

I've got this for loop here that stops the loop after the first loop due to the value of $due = 0. If value of $due is anything other than 0, the loop runs fine and everything works--problem is, this condition must always be zero. Bangin' my head against the wall on this one. Any ideas? Here's the loop:
for ($i = 0; $i < $unpaid->getInvoiceQuantity($customer_id); $i++) {
$each_invoice_id = $request->input('invoice_id' . $i);
$whats_due = DB::table('invoices')
->select('due')
->where('id','=', $request->input('invoice_id' . $i))
->first();
$invoice_payment_total = $request->input('total' . $i);
$inv_id_array = array('invoice_id' => $request->input('invoice_id' . $i), 'payment_id' => $payment_id,
'no_invoice' => false);
DB::table('payment_applications')->insert($inv_id_array);
$due = 0;
if ($whats_due->due == $invoice_payment_total) {
$due = 0;
}
if ($whats_due->due > $invoice_payment_total) {
$due = $whats_due->due - $invoice_payment_total;
}
$update_array = array('due' => $due);
DB::table('invoices')->where('id', $request->input('invoice_id' . $i))
->update($update_array);
$payment_application_update_array = array('amount' => $request->input('total' . $i));
DB::table('payment_applications')->where('invoice_id', $request->input('invoice_id' . $i))
->update($payment_application_update_array);
}

Answer 1

One of the updates must be changing the invoice quantity, and making it lower than $i, so the loop condition is no longer met.
You should save the value in a variable rather than calling the method every time.
$quantity = $unpaid->getInvoiceQuantity($customer_id);
for ($i = 0; $i < $quantity; $i++) {
// rest of loop
}