how to disable a session when there is no new user activity in a website with php?

 // Email the new password to the person.
$message = "G'Day!
 
Your personal account for the Project Web Site
has been created! To log in, proceed to the
following address:
 
http://www.example.com/
 
Your personal login ID and password are as follows:
 
userid: $_POST[newid]
password: $newpass
 
You aren't stuck with this password! You can change it at any time after you have logged in.
 
If you have any problems, feel free to contact me at <[email protected]>.
 
-Your Name
Your Site Webmaster
";
 
mail($_POST['newemail'],"Your Password for Your Website",
$message, "From:Your Name <[email protected]>");