mysql – How to edit content with a PHP session when users are logged in

I want to hide content on my index.php which should only be accessible if the user is logged in. I have code that I have learned so far:

if (isset ($ _ POST['logged']{

echo "welcome";
} other {
include "login_form.php";

I have no idea what is "recorded" in my conditional statement here. would be grateful if someone help me. Thank you