<?php
// You must first create a file "savefile_config.php" in this extensions directory and do whatever
//   checking of user credentials, etc. that you wish; otherwise anyone will be able to post SVG
//   files to your server which may cause disk space or possibly security problems
require 'savefile_config.php';
if (!isset($_POST['output_svg'])) {
    print "You must supply output_svg";
    exit;
}
$svg = $_POST['output_svg'];
$filename = (isset($_POST['filename']) && !empty($_POST['filename']) ? preg_replace('@[\\\\/:*?"<>|]@u', '_', $_POST['filename']) : 'saved').'.svg'; // These characters are indicated as prohibited by Windows

$fh = fopen($filename, 'w') or die("Can't open file");
fwrite($fh, $svg);
fclose($fh);