PHP json_decode brings back null

When your first argument is null, they're basically the same except that the null coalescing won't output an E_NOTICE when you have an undefined variable. The PHP 7.0 migration docs has this to say:

The null coalescing operator (??) has been added as syntactic sugar for the common case of needing to use a ternary in conjunction with isset(). It returns its first operand if it exists and is not NULL; otherwise it returns its second operand.

Here's some example code to demonstrate this:

<?php

$a = null;

print $a ?? 'b'; // b
print "n";

print $a ?: 'b'; // b
print "n";

print $c ?? 'a'; // a
print "n";

print $c ?: 'a'; // Notice: Undefined variable: c in /in/apAIb on line 14
print "n";

$b = array('a' => null);

print $b['a'] ?? 'd'; // d
print "n";

print $b['a'] ?: 'd'; // d
print "n";

print $b['c'] ?? 'e'; // e
print "n";

print $b['c'] ?: 'e'; // Notice: Undefined index: c in /in/apAIb on line 33
print "n";

The lines that have the notice are the ones where I'm using the shorthand ternary operator as opposed to the null coalescing operator. However, even with the notice, PHP will give the same response back.

Execute the code: https://3v4l.org/McavC

Of course, this is always assuming the first argument is null. Once it's no longer null, then you end up with differences in that the ?? operator would always return the first argument while the ?: shorthand would only if the first argument was truthy, and that relies on how PHP would type-cast things to a boolean.

So:

$a = false ?? 'f'; // false
$b = false ?: 'g'; // 'g'

would then have $a be equal to false and $b equal to 'g'.

If you go back to the main page after logging In, Try refreshing the page, If the session was set correctly then after refreshing you will be logged-In automatically or It will show you logged-In state, Else there would be something destroying all the sessions in main-page? I would go with the first condition, cause that happened to me many times, Its better to show the logIn form at the same page where you want to display registered user only content and after logging-In quickly redirect them to the same page so all the sessions work fine and back button won't create the problem as you redirected them to the same page....

EDIT: It won't effect users having there separate page as whole logIn form will be changed by that users content after logging In.

Try this:

if(isset($_SESSION['LOGGED_IN'])){
   //User is logged-In check for existence of its name file,
  $user = $_SESSION["LOGGED_IN"]."php";
If(file_exists($user)){
 //User's named file exists now include it.
  include("yourfolder/$user");
}else{
 //He was loggedIn in but file wasn't found...
 echo"Sorry nothing for you :P";
 }
}else{
   //Show the logIn form
}