Storing and retrieving _SESSION variables - php

Question

I am working on a function that allows a user to up vote a post no more than one time. There are multiple posts on a page so I need to keep track of which posts the user has voted on.
I do that by keeping a total sum of the votes casts. For instance, if a user up votes a post, they should be remove their vote by down voting once, or change their original up vote to a down vote by down voting twice.
The first time a vote is cast, I initialize the $_SESSION variable to 0. A down vote takes it to -1 and an up vote takes it to 1.
Here is the PHP function for up vote (the down vote function is similar):
function upVote($post_id) {
global $databaseController;
if(!isset($_SESSION[$post_id])) {
$_SESSION[$post_id] = 0;
}
if($_SESSION[$post_id] <= 0) {
$sql = "SELECT likes FROM posts WHERE id='" . $post_id . "'";
$results = $databaseController->select($sql);
if($results && $results->num_rows == 1) {
$results = $results->fetch_assoc();
$new_likes = ++$results['likes'];
try {
$databaseController->startTransaction();
$sql = "UPDATE posts SET likes=" . $new_likes . " WHERE id=" . $post_id;
$databaseController->update($sql);
if($databaseController->commit()) {
$_SESSION[$post_id] = ++$_SESSION[$post_id];
return true;
}
} catch (Exception $e) {
$databaseController->rollback();
return false;
}
}
}
return false;
}
The $_SESSION[$post_id] is always being reset to 0 each time the function is called allowing the user to vote as many times as they want.
Why is this happening? It seems to me that the variable should only be set to 0 if the user has never voted on the post, else it just pulls up the variable, sees that it exists, and modifies it.
Is there anything wrong with my logic? Or am I not using $_SESSION variables in the correct way?