undefined variable: article_variable in line 27

I´m currently trying to build a website with oop. For a better view I´ve created a file called Article.class.php. In this file are all functions to get and process the data. Then I´ve the second file called _header where are all instances of the class php files to work with them individualy. My last file is the “my_articles.php” file where my table gets loaded with the data of the class file. When the code gets runned and I open up the website where the table has to be, it shows me the error: undefined variable: article_variable in line 27 and Fatal error: Uncaught Error: Call to a member function my_articles() on null.

Here is the Article.class.php file

public function my_articles(){
    include_once("my_articles.php");
    
    $sql = "SELECT * FROM items ORDER BY item_name DESC";
    $statement = $this->_database_intance->prepare($sql);
    $statement->execute();
    $users = array();

    
    while ($row = $statement->fetch(PDO::FETCH_ASSOC)){
        $users[] = $row;
       
    }
    return $users;

}

Here is the _header.php file (all instances)

#Database Connection:
include('classesDatabase.class.php');
$database_variable = new Database();
$database_instance = $database_variable->connection();


#Article class instance
include('classesArticle.class.php');
$article_variable = new Article($database_instance);
$article_instance = $article_variable->poll();

include('classesUsers.class.php');
$users_variable = new Users($database_instance);
$users_instance = $users_variable->poll_users();

and my last file (my_articles.php) where the data should print out:

<tbody>
    <?php
    include_once('shared_header.php');
    $users = $article_variable->my_articles();
    foreach ($users as $row){
        ?>
        <tr>
            <td><?php echo $row['item_name']; ?></td>
            <td><?php echo $row['amount']; ?></td>
            <td>
                <a href="C:xampphtdocsAufgabeArticlesarticle_details.php?id=<?php echo $row['item_name']; ?>"class="view_button">View</a>
                <a href="edit.php?id=<?php echo $row['item_name']; ?>"class="edit_button">Edit</a>
                <a onclick="return confirm('Are you sure you want to delete this record?');" href="Articlesdelete_article.php?id=<?php echo $row['item_name'];?>"class="delete_button">Delete</a>
            </td>
        </tr>
    
        <?php
    }
    
    ?>
</tbody>