upload.php

<?php

use Office365\Runtime\Auth\UserCredentials;
use Office365\Runtime\Auth\ClientCredential;
use Office365\SharePoint\ClientContext;
use Office365\SharePoint\File;
use Office365\Runtime\Http\RequestException;

$credentials = new ClientCredential('xxxx', 'xxxxxx=');
$ctx = (new ClientContext('https://xxxx.sharepoint.com/sites/backup'))->withCredentials($credentials);


$filename = 'filename.pdf';
$fileUrl = '/sites/backup/Shared Documents/' . basename($filename);
// Check if exists
$file = $ctx->getWeb()->getFileByServerRelativeUrl($fileUrl);

// Upload
$uploadFile = $targetList->getRootFolder()->uploadFile(basename($filename),file_get_contents($filename));