The HTTP GET method should be never be used to transmit sensitive information such as credentials.
I do not see why the HTTP GET method is easier than using the HTTP POST method, see the difference in code below.
I suggest to create an API that handles the authentication and communication to the back-end server.
Since you mentioned you're going to use PHP, have a look at this API framework: http://www.slimframework.com/
Theoretically the parameters in the URL will not show up in log files since you're using HTTPS. However, there are for example, corporate proxies that perform man in the middle attacks to inspect all traffic.
In these cases, the credentials will show up in the proxy logs and this is considered a bad practice.
Here's some PHP code (which requires sanitation):
<?php
$username = $_GET['username'];
$password = $_GET['password'];
?>
This example is wrong because:
- Sensitive Information is transmitted in the URL
- No input validation has been done
This example is a little better, but still requires you to perform input validation:
<?php
$username = $_POST['username'];
$password = $_POST['password'];
?>
That's the different usage between the HTTP GET and HTTP POST method using PHP (and of course your form method should be set to POST in stead of GET as well).