Simple Visitor Online Counter

I need a Visitor Online counter. the counrt must show the number of visitor now online in the counter page. [never use the database] I need this counter without the DB. [never use any database files] just you can use the text file to record the data. I need just need to see the number of visitor on the counter page. i.e.: if just 1 person in the page show 1 and if two person in page show 2 and if one of those person leave show 1 i mean number of people now in the page. please provide demo to me and sorry about bad english.