PHP Call to undefined function

I have a method named getUserIds in a class called Db.php.

public function getUserIds() {
    $connection = $this -> connect();
    return $this -> select("select distinct user_id from users");
}

When I call the function in a file located in the same folder as Db.php I get

PHP Fatal error: Call to undefined function getUserIds()

The file generating the error looks like this:

<?php
    require('Db.php');
    $db = new Db();
    $userIds = $db -> getUserIds();
    echo '<select>';
    echo '<option value="">Choose your User Id</option>
    while($userId = mysqli_fetch_assoc($userIds)) {
        echo "<option>$userId</option>";
}
echo '</select>';
?>

Here is the contents of Db.php:

class Db {
    // The database connection
    protected static $connection;

    /**
     * Connect to the database
     * 
     * @return bool false on failure / mysqli MySQLi object instance on success
     */
    public function connect() {    
        // Try and connect to the database
        if(!isset(self::$connection)) {
            // Load configuration as an array. Use the actual location of your configuration file
            $config = parse_ini_file('./config.ini'); 
            self::$connection = new mysqli('localhost',$config['username'],$config['password'],$config['dbname']);
        }

        // If connection was not successful, handle the error
        if(self::$connection === false) {
            // Handle error - notify administrator, log to a file, show an error screen, etc.
            return false;
        }
        return self::$connection;
    }

    /**
     * Query the database
     *
     * @param $query The query string
     * @return mixed The result of the mysqli::query() function
     */
    public function query($query) {
        // Connect to the database
        $connection = $this -> connect();

        // Query the database
        $result = $connection -> query($query);

        return $result;
    }

    /**
     * Fetch rows from the database (SELECT query)
     *
     * @param $query The query string
     * @return bool False on failure / array Database rows on success
     */
    public function select($query) {
        $rows = array();
        $result = $this -> query($query);
        if($result === false) {
            return false;
        }
        while ($row = $result -> fetch_assoc()) {
            $rows[] = $row;
        }
        return $rows;
    }

    /**
     * Fetch the last error from the database
     * 
     * @return string Database error message
     */
    public function error() {
        $connection = $this -> connect();
        return $connection -> error;
    }

    /**
     * Quote and escape value for use in a database query
     *
     * @param string $value The value to be quoted and escaped
     * @return string The quoted and escaped string
     */
    public function quote($value) {
        $connection = $this -> connect();
        return "'" . $connection -> real_escape_string($value) . "'";
    }

    public function getUserIds() {
        $connection = $this -> connect();
        return $this -> select("select distinct user_id from users");
    }
}