Session variable inside a function parameter? (API CALLBACK) - php

Question

Back story: Basically I'm trying to make a BTC api & to do this I need to basically use the ($custom inside of it) as i need to parse the variable $_SESSION['username'] onto my callback file to validate the user. I'm truly stuck here.
function getAddress($address, $callback, $_SESSION["username"], $secret)
{
$username = $_SESSION['username'];
$root_url = 'https://blockchain.info/api/receive';
$parameters = 'method=create&address=' . $address .'&callback='. urlencode($callback);
$callback = $callback.'?custom='.$username.'&secret='.$secret;
$response = file_get_contents($root_url . '?' . $parameters);
$object = json_decode($response);
return $object->input_address;
}
I know that I can't just put the variable $_SESSION['username'] there however, I'm not 100% sure how to do this: callback file:
$confirmation_level = 4;
$real_secret = 'ZzsMLGKe162CfA5EcG6j'; //Your Secret Key
$address = 'address'; //Your Bitcoin Address
$username = $_GET['custom'];
$input_address = $_GET['input_address'];
$transaction_hash = $_GET['transaction_hash'];
$input_transaction_hash = $_GET['input_transaction_hash'];
$value_in_satoshi = $_GET['value'];
$value_in_btc = $value_in_satoshi / 100000000;
//skipping to validate part
$btcpaidupdate = $odb ->prepare("UPDATE `users` SET paid=1 WHERE `username` = '$username'");
$btcpaidupdate ->execute();
So as you can see I'm completely stuck on how to parse the actual variable onto the callback api file.