How to destroy / expire session after X minutes in php

function sessionTimeout($duration)
 if (isset($_SESSION['LAST_ACTIVITY']) && (time() - 

        $_SESSION['LAST_ACTIVITY'] > ($duration * 60))) {
    // last request was more than x minutes ago, where x = duration
    session_unset();     // unset $_SESSION variable for the run-time 
    session_destroy();   // destroy session data in storage
 }
}
$duration = 40; //40 minute as example
sessionTimeout($duration);
$_SESSION['LAST_ACTIVITY'] = time();