pdo - 在非对象上调用成员函数 prepare()

Fatal error: Call to a member function prepare() on a non-object in C:UsersfelVertrigoServwwwloginvalidation.php on line 42

代码:

   function repetirDados($email) {
        if(!empty($_POST['email'])) {

            $query = "SELECT email FROM users WHERE email = ?";

            $stmt = $pdo->prepare($query); // error line: line 42

            $email = mysql_real_escape_string($_POST['email']);

            $stmt->bindValue(1, $email);

            $ok = $stmt->execute();

            $results = $stmt->fetchAll(PDO::FETCH_ASSOC);

            if ($results == 0) {
                return true;
            } else {
                echo '<h1>something</h1>';
                return false;
            }
        }
    }

可能的原因是什么?另一个问题，mysql_num_rows 等价于什么?抱歉，我是 pdo 的新手