如何从其他文件访问我的变量？

I'm trying to access my variable $score from a different file but I keep getting an error message.

I've already tried adding global to it to make it a global variable and I've tried it without global.

My current code is below (not the whole files just a bit)

process.php file (lines 29 -34)

if($correct_choice == $selected_choice){
    global $score;
    $_SESSION['score']++;
    $score = $score++;
}

final.php file (lines 36 - 30)

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I want the score users received in their quiz to display on the screen, however right now the error I keep getting is 'Notice: Undefined variable: score in C:\xampp\htdocs\project\phpstaff\quiz\final.php on line 30 T' Line 30 is <?php echo $score; ?>

In case you're wondering, I've already included <?php session_start(); ?> to both files and $_SESSION['score'] seems to be working fine.