query duplicated and change data - php

Question

I am creating a coupon system for an online store using PHP (PDO queries), Mysql. Well, the system on a single page shows the coupons (table) and also creates them (form). In my BD there are 2 tables that play here in the implementation of the coupon system. (1.Table Coupons - is new) and (2.Table products - already existing before). then the products table has a last field called id_coupon and when I create a coupon I select a product from those that already exist in the store and in the products table I insert the id_coupon of the coupon in the id_coupon field of the products table located as the last column of the table and so the corresponding coupon and product correspond, likewise only if they correspond the table shows it, the problem lies in the fact that it inserts the coupon and links it perfectly in the products table (only the 1st time).
Problem: it only works in the creation of the first coupon, all linked (coupon-product with id_coupon in the 2 tables, everything is perfect), but! The second coupon created is the error (sometimes it creates 2 coupons) and other times, more than anything, it changes the id_coupon of the first coupon for the second coupon that creates a product that does not correspond and changes the corresponding id_coupon of the coupon that it was created for the previous product.
It sounds a bit confusing but that is my problem and it seems very strange to me, please I hope your collaboration and I will share the code with you (it is NOT laravel or codeigniter or object-oriented prog)
TABLE_COUPONS
TABLE_PRODUCTS
The correct solution for prevent change data to destinated product when insert a new second cupon and in this way prevent replazing first cupon id_coupon code for first product
<?php require_once('header.php'); ?>
<?php
$statement = $pdo->prepare("SELECT * FROM tbl_settings");
$statement->execute();
$result = $statement->fetchAll(PDO::FETCH_ASSOC);
foreach ($result as $row){
$row['ValorDolarUS'];
}
?>
<?php
if(isset($_POST['form1'])) {
$valid = 1;
if(empty($_POST['coupon_name'])) {
$valid = 0;
$error_message .= 'Coupon Name can not be empty<br>';
}
if(empty($_POST['coupon_start_date'])) {
$valid = 0;
$error_message .= 'Coupon Start Date can not be empty<br>';
}
if(empty($_POST['coupon_expire_date'])) {
$valid = 0;
$error_message .= 'Coupon Expire Date can not be empty<br>';
}
if(empty($_POST['coupon_code'])) {
$valid = 0;
$error_message .= 'Coupon Code can not be empty<br>';
}
if(empty($_POST['coupon_type'])) {
$valid = 0;
$error_message .= 'Coupon Type can not be empty<br>';
}
if(empty($_POST['coupon_discount'])) {
$valid = 0;
$error_message .= 'Coupon Discount can not be empty<br>';
}
if($valid == 1) {
$statement = $pdo->prepare("SELECT * FROM tbl_settings");
$statement->execute();
$result = $statement->fetchAll(PDO::FETCH_ASSOC);
foreach ($result as $row){
$row['ValorDolarUS'];
}
$ValorDolarUS = $row['ValorDolarUS'];
$CurrentPriceConvertToDolar = ($_POST['coupon_discount'] / $ValorDolarUS);
$statement = $pdo->prepare("INSERT INTO tbl_coupons(
coupon_name,
coupon_start_date,
coupon_expire_date,
coupon_code,
coupon_type,
coupon_discount,
ValorDolarUS
) VALUES (?,?,?,?,?,?,?)");
$statement->execute(array(
$_POST['coupon_name'],
$_POST['coupon_start_date'],
$_POST['coupon_expire_date'],
$_POST['coupon_code'],
$_POST['coupon_type'],
$CurrentPriceConvertToDolar,
$ValorDolarUS
));
$statement = $pdo->prepare("SELECT * FROM tbl_coupons");
$statement->execute();
$result = $statement->fetchAll(PDO::FETCH_ASSOC);
foreach($result as $row) {
$selected_product_id_coupon = $row['id_coupon'];
}
$statement = $pdo->prepare("SELECT * FROM tbl_product");
$statement->execute();
$result = $statement->fetchAll(PDO::FETCH_ASSOC);
foreach($result as $row) {
$selected_product_id = $row['p_id'];
}
$statement = $pdo->prepare("UPDATE tbl_product SET id_coupon=? WHERE p_id=?");
$statement->execute(array($selected_product_id_coupon,$selected_product_id));
$success_message = 'Coupon is created successfully and its aplied for selected product.';
$reloadpage = $_SERVER['PHP_SELF']."#CouponsForm";
header("Location:$reloadpage");
exit();
}
}
?>