Example WordPress Combination Authentication - http://wordpress.org/support/topic/integrating-wordpress-with-two-other-logins

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<?php

class StophaMultipleAuthentication
{
	// declare your active directory domain here
	CONST domain = 'your domain';
	
	/** 
	 *	Class Construct
	 *	
	 *	Hook into the WordPress Authentication procedure
	 *
	 *	@author		Nate Jacobs
	 *	@link		http://wordpress.org/support/topic/integrating-wordpress-with-two-other-logins
	 */
	public function __construct()
	{
		add_filter( 'authenticate', array( __CLASS__, 'login' ), 10, 3 );
	}
	
	/** 
	 *	Login Method
	 *
	 *	Get the values entered by the user during login. 
	 *	If the authentication method returns true, go ahead and allow access to WordPress.
	 *	But before that authenticate with HTTP Commander and HESK.
	 *
	 *	@author		Nate Jacobs
	 *	@link		http://wordpress.org/support/topic/integrating-wordpress-with-two-other-logins
         *      @param    string    $user
         *      @param    string    $user_login
         *      @param    string    $password
	 */
	public function login( $user, $user_login, $password )
	{
		$auth_result = self::can_authenticate( $user_login, $password );
		
		if ( $auth_result == true )
		{
			// add call to authenticate with HTTP Commander
			// add call to authenticate with HESK
			return new WP_User($user->ID);
		}
		else
		{
			$user = new WP_Error( 'denied', __("<strong>Log in Error</strong><br>Your password or username are incorrect.") );
			// deny them access
			remove_action('authenticate', 'wp_authenticate_username_password', 20);
			return $user;
		}
	}
	
	/** 
	 *	Actual Authentication
	 *
	 *	Take the $user_login and $password entered by the user and instead
	 *	of matching it against the WordPress DB values check it against the active directory
	 *	username/password combo.
	 *	I was able to use this to connect to the active directory at my workplace, but your
	 *	mileage may vary if it is set-up at all different. It should be enough to get your started though.
	 *
	 *	@author		Nate Jacobs
	 *	@link		http://wordpress.org/support/topic/integrating-wordpress-with-two-other-logins
         *      @param    string    $user_login
         *      @param    string    $password
         *      @return   boolean   $result
	 */
	public function can_authenticate( $user_login, $password )
	{
		if ( !$user_login )
		{
			// do nothing
		}
		else
		{
			// create the username
			$username = $user_login.'@'.self::domain;
			$ldapconn = ldap_connect( self::domain )or die("Could not connect to AD server.");
	
			$ldapbind = ldap_bind( $ldapconn, $username, $password );	
			$result = $ldapbind;
			
			// will be true or false
			return $result;
		}
	}
}