PHP headers putting everything that was output to screen into downloaded file - php

Question

I have the following PHP code using headers to download a file from the server:
$file = 'Order.txt';
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-Disposition: attachment; filename=$file");
header("Content-Type: application/zip");
header("Content-Transfer-Encoding: binary");
readfile($file);
This code works fine, however I have it in a file with the HTML form and it is ran using an if isset all of the HTML code is placed in the file, as well as anything I echo out in PHP; and the data I actually want in the file is there at the end.
I can't figure out what in the headers is causing it to write everything that is on screen to the file when downloaded. The file on he server isn't changed and is how it should be.
Thanks for any help.
Heres all the code...
<form action="<?=$_SERVER['PHP_SELF'];?>" method="post"><br>
Order Number: <input type="number" name="orderNo" size="10"/>
<input type="submit" name="Download" value="Download" />
</form>
</body>
</html>
<?php
if(isset($_POST['Download'])){download();}
function download()
{
$conn = mysql_connect('localhost', 'root', '');
mysql_select_db('amazondb', $conn);
$result = mysql_query("SELECT ID, PurchaseDate, BuyerName, ShipCity, ShipState, ShipPostalCode, ShipCountry,
ShipAddress1, ShipAddress2, ShipAddress3 FROM imported_orders");
$orderNo = $_POST['orderNo'];
$row = mysql_fetch_array($result, MYSQL_ASSOC);
if($orderNo>0&&$orderNo<=count($row))
{
$file = fopen('Order.txt', 'w');
while ($row = mysql_fetch_array($result, MYSQL_ASSOC))
{
if($row['ID']==($orderNo))
{
echo "Hello";
fwrite($file, $row['BuyerName'].PHP_EOL .$row['ShipAddress1'].PHP_EOL .$row['ShipAddress2'].PHP_EOL .$row['ShipAddress3'].PHP_EOL .$row['ShipCity'].PHP_EOL .$row['ShipState'].PHP_EOL .$row['ShipPostalCode'].PHP_EOL .$row['ShipCountry'].PHP_EOL);
}
}
fclose($file);
$file = 'Order.txt';
header("Cache-Control: private");
header("Content-Description: File Transfer");
header("Content-Disposition: attachment; filename=$file");
header("Content-Type: application/zip");
header("Content-Transfer-Encoding: binary");
readfile($file);
}
else{echo "Please enter a valid Order Number"; echo $orderNo;}
}

Answer 1

move this code at very begin of file, so you will have:
<?php
if (your condition to output file) {
$file = 'Order.txt';
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-Disposition: attachment; filename=$file");
header("Content-Type: application/zip");
header("Content-Transfer-Encoding: binary");
readfile($file);
die();
}
the rest of your file....
another solution, at the very top of file add:
<?php
ob_start() or die('Cannot start output buffering');
... your page
and just before the first header add:
$file = 'Order.txt';
ob_end_clean();
header("Cache-Control: public");
...
readfile($file);
die();