Gets posts_per_page via $_COOKIE

I seem to not be able to understand why a) my variable is not being returned within the page and, b) why there seems to be a cookie set for each individual page.

AIM:

In a product listing, I have the option to filter results by (small numbers for this example) 5, 10, or all. When I paginate the $_COOKIE resets to the default value, and once set is set for that page. I can alternate between page 1 and page 2 with different listings of results.

File "cookies.php" (before <!doctype html>):

<?php $post_per_page = $_POST['post_per_page'];
    if (isset($post_per_page)) {
        // Store it in the cookie
        setcookie('post_per_page', $post_per_page, time()+60*60*24, "/");
        // And save in a variable
        $postsperpage = $post_per_page;
    }

    // Then check existing cookies
    else if (isset($_COOKIE['post_per_page'])) {
        $postsperpage = $_COOKIE['post_per_page'];
    }

    // Finally use a default value if none was set
    else {
        // Default to 3 for example:
        $postsperpage = 3;
    }
?>

Within my taxonomy.php:

$args = array(
    'post_type'         =>      'product',
    'posts_per_page'    =>      $postsperpage,
    'paged'             =>      $paged
);

In this the $postsperpage is empty, not even returning the default "3" from the cookies.php. So I have to recall the contents of cookies.php commenting out the setcookie line.

Markup:

<form method="post">
    <button type="submit" name="post_per_page" value="5">5</button>
    <button type="submit" name="post_per_page" value="10">10</button>
    <button type="submit" name="post_per_page" value="-1">ALL</button>
</form>

EDIT

Somehow the cookie setting has fixed itself up. But I am still confused why I have to include the cookies.php contents into the taxonomy rather than its initial variable setting being global.

Solution

Unless these php files are being included on the same page, the $postsperpage variable doesn't exist on taxonomy.php. If that is the case, you should just retrieve the cookie value on that page like so...

$postsperpage = $_COOKIE['post_per_page'];