Check PayPal Payment Is completed - php

Question

I have problems to make my PayPal system payment.
1. User add on cart the elements him want. (In this example, user select 3 items: PDF - HTML, PDF -JS and PDF -JS)
2. In page when appear that 3 items and button to pay (shoppingCart.php), have the next code:
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="business" value="myEmail#domain.com">
<input type="hidden" name="currency_code" value="EUR">
<input type="hidden" name="item_name_1" value="PDF - HTML">
<input type="hidden" name="amount_1" value="5">
<input type="hidden" name="item_name_2" value="PDF - JS">
<input type="hidden" name="amount_1" value="7">
<input type="hidden" name="item_name_3" value="PDF - CSS">
<input type="hidden" name="amount_1" value="10">
<input type="hidden" name="return" value="PaymentCompleted.php">
<input class="btn btn-primary mt-2 m-b-20 " type="submit" value="PayNow">
<form>
3. When user completes payment, on PayPal windows, click to "Return to Shop" and go to page "PaymentCompleted.php".
4. On "PaymentCompleted.php" have a SQL script who add that 3 items on clientArea payments.
Problem I have, are the next:
If user add that 3 items, and go to PaymentCompleted.php, SQL Script add to it ClientArea without do payment.
How I can check if PayPal payment has been completed and else, prevent SQL Script add that items to clientArea?
Have a good day