Cannot pass value from form to variable to be posted - php

Question

I want to pass some value from my form to be sent with POST. However, when i use dropdown, it wont detect the value and thus giving me undefined index error for every variable
When i use simple text form to post, it works, but i need to use the dropdown form.
dropdown for shop code
<?php $stmt = $shop->readName();
echo "<select class='form-control name='shop_id'>";
echo "<option>select shop name</option>";
while ($row_shop = $stmt->fetch(PDO::FETCH_ASSOC)) {
extract($row_shop);
echo "<option value='{$shop_id}'>{$shop_name} </option>";
}
echo "</select>";
?>
and the post code
if ($_POST) {
// Set values
$transaction - > customer_id = $_POST['customer_id'];
$transaction - > shop_id = $_POST['shop_id'];
$transaction - > staff_id = $_POST['staff_id'];
// create transaction
if ($transaction - > add()) {
echo "<div class='alert alert-success'>Transaction was created.</div>";
}
// if unable to create the staff, tell the user
else {
echo "<div class='alert alert-danger'>Failed.</div>";
}
}
EDIT : Now the error is gone, but the form simply wont do anything :(
Here's the screenshot of the form
form

Answer 1

Undefined index errors means that you haven't defined $transaction as an object, thus when you try to update one of the values (for example $transaction->customer_id), PHP will give you an error.
Try defining $transaction like so:
$transaction = new yourObjectNameHere();
Just after the if($_POST).