Why is my form not being submitted? - php

Question

I have 2 functions - one generates the form on my main page and the other processes the submitted form. This is the Braintree sandbox API and their method is this: take in user info and submit to Braintree server, BT server returns a payment method nonce to me which I can then use to POST and view the transaction in my sandbox control panel. However, the form isn't being submitted and I'm not sure at what point in the process the whole submission is failing. NOTE - I am submitting the form to the same PHP file where the form is located.
I still need help on this...
ask.php - This is the page where I call both functions
<div>
<?php
fd_bt_form();
fd_process_trans();
?>
</div>
find-do-for-anspress.php
$FD_Braintree_Keys = array(
Braintree_Configuration::environment('sandbox'),
Braintree_Configuration::merchantId('A'),
Braintree_Configuration::publicKey('B'),
Braintree_Configuration::privateKey('C')
);
function fd_bt_form()
{
$class_bt_token = new Braintree_ClientToken();
$clientToken = $class_bt_token->generate();
?>
<script src="https://js.braintreegateway.com/v2/braintree.js"></script>
<script>
braintree.setup(
'<?php echo $clientToken ?>',
'custom', {
id: 'checkout',
});
</script>
<?php
echo
'<form id="checkout" action="" method="POST">
<p>
<label><font size="5">Amount:</font></label>
<input type="text" size="4" name="amount" id="amount" />
</p>
<input data-braintree-name="number" value="378282246310005">
<br> <br />
<input data-braintree-name="expiration_month" value="05">
<input data-braintree-name="expiration_year" value="17">
<br> <br />
<input data-braintree-name="cvv" value="531">
<br> <br />
<input type="submit" id="submit" value="Pay">
</form>';
echo $_POST["payment_method_nonce"];
global $bt_nonce;
$bt_nonce = $_POST["payment_method_nonce"];
return $bt_nonce;
}
function fd_process_trans() {
$FD_Braintree_Keys;
$nonce = $_POST["payment_method_nonce"];
$amount = $_POST["amount"];
$result = Braintree_Transaction::sale(array(
'amount' => $amount,
'paymentMethodNonce' => $nonce,
'options' => array(
'submitForSettlement' => True,
),
));
if ($result->success) {
echo "Success!";
}
else {
echo "Transaction failed.";
}
}