While ngrok can be used to expose your local development application to the internet for testing, 
the free version of ngrok does not support End-to-End TLS Tunnels aka https, but it does support forwarding of https to http

Note: Transport Layer Security (TLS) is the successor to SSL. TLS 1.0 was defined in RFC 2246 in January 1999.  Hypertext Transfer Protocol Secure (HTTPS), or “HTTP Secure,” is an application-specific implementation that is a combination of the Hypertext Transfer Protocol (HTTP) with the SSL/TLS.
reference: is-it-ssl-tls-or-https

For Single Sing On solutions, such as SAML, you application is required to be hosted over https

You might think you could just bind to the ssl port 443

> ngrok http myawesomeapp.local:443

Session Status                online
Session Expires               7 hours, 4 minutes
Version                       2.2.8
Region                        United States (us)
Web Interface       
Forwarding                    http://8316dcb8.ngrok.io -> myawesomeapp.local:443
Forwarding                    https://8316dcb8.ngrok.io -> myawesomeapp.local:443 

But, accessing https://8316dcb8.ngrok.io or http://8316dcb8.ngrok.io results in the browser showing

Bad Request
Your browser sent a request that this server could not understand.
Reason: You're speaking plain HTTP to an SSL-enabled server port.

So that is why you need the paid version of ngrok for End-to-End TLS Tunnels.

One solution is to pay for the Pro version of ngrok, which also gets you access to more features such as Whitelabel domains, Reserved TCP addresses, End-to-End TLS Tunnels, and more resources.

However, if your using PHP Symfony or the PHP OneLogin SAML library,
another solution is to modify your application to indicate that the request is actually https.

Start up ngrok

> ngrok http myawesomeapp.local:80

Session Status                online
Session Expires               7 hours, 4 minutes
Version                       2.2.8
Region                        United States (us)
Web Interface       
Forwarding                    http://8316dcb8.ngrok.io -> myawesomeapp.local:80
Forwarding                    https://8316dcb8.ngrok.io -> myawesomeapp.local:80  

If you view https://8316dcb8.ngrok.io, you will see your app, but you app will think its running on port 80 and http

Note: While this is PHP focused, the concept should apply to other applications.

To make your app think it's on https, in the bootstrap or initialization of your application, add:

// If the host url has ngrok.io, and the header X-Forwarded-Proto set by ngrok is https, then also set https on and the port to 443

if (strpos($_SERVER['HTTP_HOST'], 'ngrok.io') !== false && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')


  $_SERVER['HTTPS'] = 'on';

  $_SERVER['SERVER_PORT'] = 443;


Note: For Symfony, you can place this code within the boostrap app_dev.php, above the creation of $request

> vi app_dev.php

// place here

$request = Request::createFromGlobals();
$response = $kernel->handle($request);

This will allow any following code which checks for https 

such as OneLogins Utils::isHTTPS()
or Symfony Request->isSecure()

to return true when accessing the ngrok url over https

Note: This does not test or affect your applications ssl certs, as the browser thinks the request is from 8316dcb8.ngrok.io,  And if you are using ngrok daily for development or testing, it is fairly inexpensive to upgrade to the tls version which is a  simpler and better solution

Hopefully this helps you test Single Sing On solutions, such as SAML.

-End of Document-
Thanks for reading