Heroku PHP app getting 302 error on some links, not others - apache

Question

I'm migrating a working PHP app from my localhost, XAMPP, to Heroku and a few of my links give me a 302 error and the load page says:
"This page isn’t working
mywebsite.heroku.com sent an invalid response.
ERR_INVALID_REDIRECT"
Here is my code for that particular link:
Admin
Here's the code for another broken link that does the same thing:
Login/Register
HOWEVER, here is the code for the link directly beneath that one that works perfectly:
View Cart
Here's the code that has that variable defined.
$web_url = "https://" . $_ENV["HTTP_HOST"] . "/";
EDIT: I've narrowed it down to links that take users to pages where data is entered. It's an ecommerce website so account registration, login, admin login, and checkout where the user enters billing info and credit card.
I think it has to do with the apache server. I do -not- have an .htaccess file so there's nothing in there that would cause this redirect.
Any ideas are appreciated. I am trying to avoid spending a week reading the entire apache documentation.

Answer 1

would suspect the problem to be here...
$app_path = '/' . $dirs[1] . '/' . $dirs[2] . '/';
which should rather be (due to zero-based indexes):
if(sizeof($dirs) == 2) {
$app_path = '/' . $dirs[0] . '/' . $dirs[1] . '/';
}
better rewrite with mod_rewrite, before the request hits any script.
... and when developing on the localhost, xdebug is a good tool.