PHP file stops working with no changes made to it - php

Question

I am trying to get a website to work, but cannot figure out how to edit the server PHP file without the website crashing. I am not getting many hits on google looking for this issue.
I am attempting to build a "stripe checkout" page, where you can put a hold on a card. I did not write any of this code, but got it on github.
https://github.com/stripe-samples/placing-a-hold
The index.php file is what I am trying to edit, but this is where I am having trouble:
https://github.com/stripe-samples/placing-a-hold/tree/master/using-webhooks/server/php
If I download the github link and open the php file through my browser, I get a checkout page as expected. However, if I copy and paste the file on my desktop, open it, save it (no edits), close it, and re-upload it to the server, it gives me a white screen. If I re-upload the original php file it works again. I dont see how two identical php files can give me this result. I was thinking that saving the php file changes the permissions associated with the file? but I did a chmod 777 to the entire directory, and it still doesn't work.
I don't have much coding experience. I checked the server error it says:
PHP Fatal error: Uncaught RuntimeException: Unexpected data in output buffer. Maybe you have characters before an opening <?php tag? in /var/www/html/server/php/vendor/slim/slim/Slim/App.php:625Stack trace:#0 /var/www/html/server/php/vendor/slim/slim/Slim/App.php(333): Slim\App->finalize(Object(Slim\Http\Response))#1 /var/www/html/server/php/index.php(112): Slim\App->run()#2 {main}\n thrown in /var/www/html/server/php/vendor/slim/slim/Slim/App.php on line 625,
going into the App.php file around line 625 it has this:
protected function finalize(ResponseInterface $response)
{
// stop PHP sending a Content-Type automatically
ini_set('default_mimetype', '');
$request = $this->container->get('request');
if ($this->isEmptyResponse($response) && !$this->isHeadRequest($request)) {
return $response->withoutHeader('Content-Type')->withoutHeader('Content-Length');
}
// Add Content-Length header if `addContentLengthHeader` setting is set
if (isset($this->container->get('settings')['addContentLengthHeader']) &&
$this->container->get('settings')['addContentLengthHeader'] == true) {
if (ob_get_length() > 0) {
throw new \RuntimeException("Unexpected data in output buffer. " .
"Maybe you have characters before an opening <?php tag?");
}
$size = $response->getBody()->getSize();
if ($size !== null && !$response->hasHeader('Content-Length')) {
$response = $response->withHeader('Content-Length', (string) $size);
}
}
// clear the body if this is a HEAD request
if ($this->isHeadRequest($request)) {
return $response->withBody(new Body(fopen('php://temp', 'r+')));
}
return $response;
}