Have the form action parameter (<form action="" method="post">) be the name of the same file, or rather $_SERVER['PHP_SELF']. This will load the POST data to the same file where you can process it and send the data to yourself in an email. Afterwards, you can redirect the user to Paypal.
Ex.
<?php
// Check if the form was submitted. (Should have a submit-type button with the name "submit" in your form
if(isset($_POST['submit'])){
// Validate submitted data and send email to yourself
// Now, redirect to Paypal
header("Location: http://www.paypal.com?param1={$_POST['param1']}");
exit();
}
?>
You can tack on any necessary parameters to the redirect URL, as I demonstrated by adding ?param1={$_POST['param1']}