[php] Fatal error: Call to a member function prepare() on null

I am trying to access a list of categories and their contents. I have a class called Categories. I keep getting this error. The weird thing is that I've used this same exact code in two other places so far with no problems. All I did here was reuse the code and change all the variables.

Fatal error: Call to a member function prepare() on null

Here is the code to my class:

    <?php

class Category {
    public function fetch_all() {
        global $pdo;

        $query = $pdo->prepare("SELECT * FROM dd_cat");
        $query->execute();

        return $query->fetchAll();
    }

    public function fetch_data($cat_id) {
        global $pdo;

        $query = $pdo->prepare("SELECT * FROM dd_cat WHERE cat_id = ?");
        $query->bindValue(1, $cat_id);
        $query->execute();

        return $query->fetch();
    }
}

?>

And here is the code I am trying to call:

<?php
session_start();
//Add session_start to top of each page//
require_once('includes/config.php');
require_once('includes/header.php');
include_once('includes/category.php');

?>
<link rel="stylesheet" href="css/dd.css">
    <div id="menu">
        <a class="item" href="drop_index.php">Home</a> -
        <a class="item" href="create_topic.php">Create a topic</a> -
        <a class="item" href="create_cat.php">Create a category</a>
        <div id="userbar">
<?php
    if( $user->is_logged_in() )
    {
        echo 'Hello ' . $_SESSION['user_name'] . '. Not you? <a href="logout.php">Sign out</a>';
    }
    else
    {
        echo '<a href="login.php">Sign in</a> or <a href="index.php">create an account</a>.';
    } 
?>
        </div>
    </div>

<?php

$category = new Category;
$categories = $category->fetch_all();

?>
    <div id ="wrapper">
        <h1>Categories</h1>
        <section>
            <ul>
                <?php foreach ($categories as $category) { ?>
                    <li><a href="category.php?id=<?php echo $category['cat_id']; ?>">
                        <?php echo $category['cat_title']; ?></a> 
                    </li>
                <?php } ?>
            </ul>
        </section>
    </div>
<?php
require_once('includes/footer.php');
?>

@delato468 comment must be listed as a solution as it worked for me.

In addition to defining the parameter, the user must pass it too at the time of calling the function

fetch_data(PDO $pdo, $cat_id)

In ---- model: Add use Jenssegers\Mongodb\Eloquent\Model as Eloquent;

Change the class ----- extends Model to class ----- extends Eloquent

You can try/catch PDOExceptions (your configs could differ but the important part is the try/catch):

try {
        $dbh = new PDO(
            DB_TYPE . ':host=' . DB_HOST . ';dbname=' . DB_NAME . ';charset=' . DB_CHARSET,
            DB_USER,
            DB_PASS,
            [
                PDO::ATTR_PERSISTENT            => true,
                PDO::ATTR_ERRMODE               => PDO::ERRMODE_EXCEPTION,
                PDO::MYSQL_ATTR_INIT_COMMAND    => 'SET NAMES ' . DB_CHARSET . ' COLLATE ' . DB_COLLATE

            ]
        );
    } catch ( PDOException $e ) {
        echo 'ERROR!';
        print_r( $e );
    }

The print_r( $e ); line will show you everything you need, for example I had a recent case where the error message was like unknown database 'my_db'.