вставить несколько строк на основе условия if

Я возвращаю массив результатов, которые помещаю в цикл foreach, и каждый результат сравнивается с одной суммой. Пока условие выполнено, оно должно вставлять строки. Тем не менее, никакие данные никогда не вставляются в базу данных, но журналы ошибок также пусты, т.е. я не получаю ошибок.

        foreach ($invoices as $invoice) {
           if ($invoice->amount <= $this->amount) {
               $stmt = $db->prepare("INSERT INTO 'payment_link' ('amount') VALUES (:amount)");
               $stmt->bindValue(":amount", $invoice->amount, PDO::PARAM_STR);
               $stmt->execute();
           }
        }

print_r($invoices) дает мне:

Array ( [0] => App\Models\Invoice Object ( [customer_id] => [description] => [date_created] => [amount] => 300.00 [total:App\Models\Invoice:private] => [invoice_id] => [line_description] => [line_amount] => [errors] => Array ( ) ) [1] => App\Models\Invoice Object ( [customer_id] => [description] => [date_created] => [amount] => 500.00 [total:App\Models\Invoice:private] => [invoice_id] => [line_description] => [line_amount] => [errors] => Array ( ) ) ) Array ( [0] => App\Models\Invoice Object ( [customer_id] => [description] => [date_created] => [amount] => 300.00 [total:App\Models\Invoice:private] => [invoice_id] => [line_description] => [line_amount] => [errors] => Array ( ) ) [1] => App\Models\Invoice Object ( [customer_id] => [description] => [date_created] => [amount] => 500.00 [total:App\Models\Invoice:private] => [invoice_id] => [line_description] => [line_amount] => [errors] => Array ( ) ) )