<?php
/*
Plugin Name: Ozh's Simple Live Visitor Counter
Plugin URI: http://planetozh.com/blog/2010/05/overview-of-wordpress-transients-api/
Description: Display IPs of last 10 visitors
Author: Ozh
Version: 1.0
Author URI: http://ozh.org/
*/

// Get visitor IP, update and return list
function wp_ozh_slvc() {
	$ip = $_SERVER['REMOTE_ADDR'];
	
	$visitor_ips = get_transient( 'ozh_livecounter' );
	
	if( false == $visitor_ips OR !in_array( $ip, $visitor_ips ) ) {
		// our transient has expired (or has never been set) or doesn't contain this IP
		$visitor_ips[] = $ip;

		// just keep at most last ten entries
		if( count( $visitor_ips ) > 10 )
			array_shift( $visitor_ips );
		
		// store this transient with a 3 minutes expiration
		set_transient( 'ozh_livecounter', $visitor_ips, 60*3 );
	}
	
	return join( ', ',  $visitor_ips );
}

/*	Usage in template file:
	echo "currently online: ".wp_ozh_slvc();
*/