Using the Back button on your browser causes it to display the copy of the web page it has in it's cache. The PHP and/or Javascript code behind the page does not run again.
When you do as JB suggested...
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					Originally Posted by  johnnybax
					 
				 
				Typically, once you're done reading a thread if  you will scroll to top of page and click the forum instead of just  clicking the back button on your browser it will mark a thread as  read. 
			
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You cause the browser to get a new copy of the web page from the site's server. This also causes the PHP and/or Javascript code behind the page to execute again, updating the information the web page displays.