//call add_filter or add_action inside a class
add_filter('posts_where', array( &$this, 'filter_where' ));
 
 
//example
class IP_Ban {
 
	function IP_Ban()
	{
		$this->__construct();
	} // function
 
	function __construct()
	{
		new IP_Ban_Options;
 
		add_action( 'admin_init', array( &$this, 'admin_init' ) );
		add_action( 'init', array( &$this, 'init' ) );
	} // function
 
	function admin_init()
	{
		# perform your code here
	} // function
 
	function init()
	{
		# perform your code here
	} // function
 
}