Using a global array inside a class
$CONFIG = array();
// ...
$CONFIG["DATABASE"] = array();
$CONFIG["DATABASE"]["USERNAME"] = "user";
$CONFIG["DATABASE"]["PASSWORD"] = "pass";
$CONFIG["DATABASE"]["HOSTNAME"] = "127.0.0.1";
$CONFIG["DATABASE"]["DATABASE"] = "my_db";
// ...

require('config.php');

class Database
{
    protected   $m_Link;
    private     $m_User;
    private     $m_Pass;
    private     $m_Host;
    private     $m_Data;
    private     $m_bConnected;

    public function __construct()
    {
        global $CONFIG;
        $this->m_User = $CONFIG["DATABASE"}["USERNAME"]; // Line #16
        $this->m_Pass = $CONFIG["DATABASE"}["PASSWORD"];
        $this->m_Host = $CONFIG["DATABASE"}["HOSTNAME"];
        $this->m_Data = $CONFIG["DATABASE"}["DATABASE"];
        $this->m_bConnected = false;
        $this->Connect();
    }

    // ...
};

$this->m_User = $CONFIG["DATABASE"}["USERNAME"];
----------------------------------^

$this->m_User = $CONFIG["DATABASE"]["USERNAME"];