Personally I think the best implementation is to use the Singleton design pattern and not the global keyword in this instance. So your code for MYSQLDB class would be:

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class MYSQLDB
{
  private static instance = null;
  // private constructor and clone methods
  private __construct()
  {
    // initialise any variables here
  }
  private __clone()
  {
  }
  public static function getInstance()
  {
    // this code ensures there is only ever at most one instance of this class
    if (instance == null)
        instance = new MYSQLDB();
    return instance;
  }
  public function validateLogin($member, $password)
  {
     // check the member/password details in the database...
  }
}

class MYSQLDB { private static instance = null; // private constructor and clone methods private __construct() { // initialise any variables here } private __clone() { } public static function getInstance() { // this code ensures there is only ever at most one instance of this class if (instance == null) instance = new MYSQLDB(); return instance; } public function validateLogin($member, $password) { // check the member/password details in the database... } }

Then in your session class you call the getInstance method to initialise your database accessor like so:

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class session
{
  private $database;
  public __constuct()
  {
    $this->database = MYSQLDB::getInstance();
  }
  function login($member, $password)
  {
    // now you can use your database variable like so:
    $this->database->validateLogin($member, $password);
  }
}

class session { private $database; public __constuct() { $this->database = MYSQLDB::getInstance(); } function login($member, $password) { // now you can use your database variable like so: $this->database->validateLogin($member, $password); } }

I believe this method is cleaner than the global approach, but I'll leave it up to you to decide what's best for your application.

If the $instance variable is null, it is created then returned. If it is not null, it is simply returned without being re-created. Your example is equivalent however. Note that my code does not have curly braces for the if-statement, so only the first line after the condition is skipped if the condition is false.