Exporting Large Google Files Via API - php

Question

I'd like to know how to perform a large Google Slide export via API. I've followed the instructions here and it works fine with small files. If I try it with a large file, it just doesn't work. It is not returning errors or anything. The logic I'm using is the following:
<?php session_start();
ini_set('max_execution_time', 300); //300 seconds = 5 minutes
//INCLUDE PHP CLIENT LIBRARY
require_once 'vendor/autoload.php';
// Create client object
$client = new Google_Client();
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/main.php');
$client->setAuthConfig("client_secret.json");
$client->addScope(array('https://www.googleapis.com/auth/drive'));
$client->setAccessType('offline');
if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
$client->setAccessToken($_SESSION['access_token']);
$service = new Google_Service_Drive($client);
$pptFile = fopen("testPresentation.pptx", "w+");
$fileId = '1fx2uqe96NgsOabAv8S_mItlEbJ_NJSORqPitH55TMc';
try {
$file = $service->files->export($fileId,"application/vnd.openxmlformats-officedocument.presentationml.presentation", array("alt" => "media"));
$fileContent = $file->getBody()->getContents();
fwrite($outHandle, $fileContent);
} catch(Exception $e) {
print $e->getMessage();
}
//Close output file handle.
fclose($outHandle);
echo "Done.\n";
} else {
if (! isset($_GET['code'])) {
$auth_url = $client->createAuthUrl();
header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else {
$client->authenticate($_GET['code']);
$_SESSION['access_token'] = $client->getAccessToken();
$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/main.php';
header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}
}
Like I said, it works with small files but not with large ones. One thing I've noticed is that when I do a var_dump($file); on the small file, the 'Content-Length' header has a size of "30083".
If I do the same on the large file, the 'Content-Length' header has a size of "0". I assume that exporting large files must be done different. I even run into this https://github.com/google/google-api-php-client/issues/1124 and tried the snippet there but had no success.