데이터베이스 연결에서 \'null에서 prepare() 멤버 함수 호출\' 오류를 해결하는 방법은 무엇입니까?

<code class="php"><?php

// Instantiate a new PDO object globally
$pdo = new PDO('mysql:host=localhost;dbname=test', $user, $pass);

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

        // Use the initialized $pdo variable to prepare the SQL query
        $query = $pdo->prepare("SELECT * FROM dd_cat");
        $query->execute();

        return $query->fetchAll();
    }

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

        // Use the initialized $pdo variable to prepare the SQL query
        $query = $pdo->prepare("SELECT * FROM dd_cat WHERE cat_id = ?");
        $query->bindValue(1, $cat_id);
        $query->execute();

        return $query->fetch();
    }
}

?></code>