Hi Guys,
I am very new to web development and particular PHP. I have been developing fat applications for the last 12 years, so I am not so new to programming.

Earlier today I started creating a very simple login form, that used the action="post" in the <form> tag to send the captured details to a waiting PHP page.
This works like intended and I managed to connect to my DB and successfully validate the login details.

Then later today, reading w3schools material I came across PHP Form validation using the action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" action and I applied the code samples they provided, and that works really, really well.
And right here is my conundrum. I know how to get data from one page to another with a normal action="waiting_page.php", but when using the sample above, I actually don't know how to move to the awaiting page once my form validation was successful.
I have gone through the w3schools material a couple of times and cannot figure out how to direct the call to the intended page and passing the values from the form. It must be very obvious (is my only conclusion) because it is not stated there, but I am lost.
It has been several hours and I would really appreciate the line of code that I am missing...

thank you,
Dawid