javascript : send file to php script for user to download - php

Question

In the website I'm building, I'm trying to provide users the ability to export and download data in KML format. I've built a PHP script to generate a kml file from a string. It works reasonably well, until requests get too long and then I get 414 errors.
The thing is, I'm pretty sure I'm going at this the wrong way (I'm new to php). Rather than sending my data as a string, which can get to multiple tens of thousands of characters long, I should be sending a file generated by javascript to the php script which would send it back to the user or something like this. Is it possible?
If not, what other options do I have?
Here's my php script :
if($_SERVER['REQUEST_METHOD']=='POST') {
$text = $_POST['text'];
$text = str_replace('\r\n', PHP_EOL, $text);
$text = str_replace('\n', PHP_EOL, $text);
$text = str_replace('\t', "\t", $text);
$text = str_replace('_HASH_', "#", $text);
$filename = $_POST['filename'];
$tmpName = tempnam(sys_get_temp_dir(), $filename);
$file = fopen($tmpName, 'w');
fwrite($file, $text);
fclose($file);
header('Content-Description: File Transfer');
header("Content-Type: application/octet-stream");
//header('Content-Type: text/plain');
header('Content-Disposition: attachment; filename=' . $filename . '.kml');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($tmpName));
ob_clean();
flush();
readfile($tmpName);
unlink($tmpName);
exit;
}
* EDIT *
Ok, I changed the php script to use POST instead of GET which should clear up the length issue. But this has shown me another problem.
I'm using dojo to write my website. I'm doing an xhr request with POST method. This request comes back successfully. In the success handler, I'm navigating to the same url I passed to the request by setting the src of a hidden iFrame (so as to avoid reloading the page). The results in a GET request which doesn't work with my script (POST).Furthermore, I believe this means I'm running the php script twice which makes no sense.
Should I just navigate directly to the url of the php script with required parameters? In this case, how do I navigate to a url using a POST request?
I'm a bit confused about all of this, it probably shows too!
Thanks,
* SOLUTION *
mguimard sent me on the right track to solve my issue. Here's the bit of code I used to send my POST request while avoiding to realod the page:
download: function(text) {
var ifr = document.createElement('iframe');
ifr.setAttribute('name', "target");
ifr.style.display = 'none';
document.body.appendChild(ifr);
var f = document.createElement('form');
f.setAttribute('method',"post");
f.setAttribute('action',"saveFileAs.php");
f.setAttribute('target',"target");
addField(f, "filename", "myFileName");
addField(f, "text", text);
f.submit();
ifr.onload = cleanUp;
function cleanUp(){
document.body.removeChild(ifr);
ifr = null;
}
function addField(form, name, value) {
var i = document.createElement("input");
i.setAttribute('name',name);
i.setAttribute('type','text');
i.value = value;
form.appendChild(i);
}
}
Ggilmann