Subscribing a user to a category

[TechnoDiver]

Hi Freaks, I've been trying to put together subscribe functionality on a project. Been at it for hours and can't make any progress.

The categories that a user is subbed to are stored in comma separated string in the database and pulled out into $_SESSION['user_subs']

I have these 2 methods here to start ->

<?php
private function userSubscribed($cat_id) {
    $category = $this->getCategoryByID($cat_id);
    if(in_array($category, $_SESSION['user_subs'])) { 
        echo "user subscribed";
        return true;
    } else {  
        echo "user NOT subscribed"; 
        return false;       
    }
}

public function displaySubscribedButton($cat_id) {
    $subscribed = $this->userSubscribed($cat_id);
    if($subscribed) {
        $action_btn = "<td><a href='category_posts.php?cat_id=$cat_id' name='sub_btn' class='btn btn-secondary'>Unsubscribe</a>";
    } else {
        $action_btn = "<td><a href='category_posts.php?cat_id=$cat_id' name='sub_btn' class='btn btn-danger text-color-									primary'>Subscribe</a>";
    }

    return $action_btn;
}

They've both been well tested and I know they're working correctly.

Then I have this one (which is the problem child) ->

<?php
public function updateSubscriptionCategory($cat_id, $username) {
    $category = $this->getCategoryByID($cat_id);
    $subscribed = $this->userSubscribed($cat_id);

    print_r($_SESSION['user_subs']);

    if($subscribed){
        unset($_SESSION['user_subs'][$category]);
        $subs = $_SESSION['user_subs'];
        $subscribed = false;
    } else {
        $new_sub = ",".$category;
        $subs = array_push($_SESSION['user_subs'], $new_sub);
        $subscribed = true;
    }

    $stmt = $this->conn->prepare("UPDATE ".User::$table." SET subs=? WHERE username=?");

    if($stmt) {
        $stmt->execute([$subs, $username]);
    } else {
        echo "you fucked up";
    }
}

I call it from the category file, here ->

<?php require_once("assets/initializations.php"); 
$post_obj = new Post($conn, $username);
$cat_obj = new Category($conn, $username);

$cat_id = $_GET['cat_id'];
if(isset($_POST['sub_btn'])) {
    $cat_obj->updateSubscriptionCategory($cat_id, $username);
    // header("Location: category_posts.php?cat_id=$cat_id&cat_title=$cat_title");
}

but the page only reloads no changes are made not on the page nor in the db nor do I get any feedback, not from the 'print_r' or the else at the bottom. I feel like it's not even running this method. Was hoping that someone sees something that I've been missing. 8 hours in and I'm still not able to get it to work. TIA for all input

[ginerjm]

Proper database design would never use a comma-separated list of items in a field.  It would be done as a group of records in a different table that holds the user's id number to connect them all to that subscriber.