php session logic error - php

Question

I am doing a tictactoe on php for homework, and I am stuck on the logic that changes from player to player, as it always put a X on the cell and never changes to O whenever the submit button is hit. My question is about how to make it work.
What I have so far for this is:
$position = filter_input(INPUT_GET, 'Position');
$player = "C_$position";
//Problem
if(!isset($_SESSION[$player])){ //Also tried the function empty before using !isset
$_SESSION[$player]="X";
}
if($_SESSION[$player]=="X"){
$_SESSION[$player]="O";
}
if($_SESSION[$player]=="O"){
$_SESSION[$player]="X";
}
//END PROBLEM
I've been looking for answers and I found this Initialize the variable only once in php however it doesn't work as I applied it to my php code. If anyone got a solution I would totally appreciate it.

Answer 1

Use an else rather than another if because with the second if you revert to the original value. Currently you run into a TRUE conditional chain, every conditional is met so you end up with the last conditional's value. A rough demo of your code can be seen here, https://3v4l.org/Lv7CK.
This should correct your logic.
$position = filter_input(INPUT_GET, 'Position');
$player = "C_$position";
if(empty($_SESSION[$player]) || $_SESSION[$player]=="O"){
$_SESSION[$player]="X";
} else {
$_SESSION[$player]="O";
}
This should set the player to X if the session isn't set or if the current player is O, otherwise it sets the player to O (this assumes there are only two values for $player if there are multiples use an elseif, or possibly a switch).