Updating text field globally

Basically what I need is a button that changes a dynamic text field on my website, and is viewable globally by all users.

For example Joe see’s the # 5 in this text field. He clicks it, and the number then updates to a 4. Another user can come by and click it later and it will go down to 3. Etc Etc.

I know this can be done using php/variables…but not sure how exactly. Any ideas?