The PHP function to perform an SQL query does not work

I am trying to use a function to perform an SQL query and return the result. Doing a manual SQL query with the same query as the below works and returns the correct result but doing it within PHP is not working at all (page is blank).

connection.php

<?php

require_once("settings.php");

$conn = oci_connect($db_user, $db_pass, $db_ip.'/'.$db);
if (!$conn) {
    $e = oci_error();
    trigger_error(htmlentities($e['message'], ENT_QUOTES), E_USER_ERROR);
}

?>

functions.php

<?php

require_once("connection.php");

function GetInvoice($n)
   // returns invoice number for given order number
{
   $stid = oci_parse($conn, 'SELECT InvoiceNo FROM Orders WHERE OrderNo = $n');

   oci_define_by_name($stid, 'InvoiceNo', $InvoiceNo);

   oci_execute($stid);

   echo "$InvoiceNo";
} 

GetInvoice(999645);

?>

Doing the manual SQL query shows a resulting InvoiceNo, it is just not working in PHP.

Thanks

Steve

---

You are trying to use a variable outside of function inside the function. That variable is not in function's scope. Due to this, $conn is undefined. Also, you have used $InvoiceNo withoud fetch result, I did some refoctoring on that place. You can use following;

function GetInvoice($n, $conn)
   // returns invoice number for given order number
{
   $stid = oci_parse($conn, 'SELECT InvoiceNo FROM Orders WHERE OrderNo = $n');

   oci_define_by_name($stid, 'InvoiceNo', $InvoiceNo);

   oci_execute($stid);

   while (oci_fetch($stid)) {
       echo "$InvoiceNo";
   }
}

GetInvoice(999645, $conn);