You can unset session variable using:

1. session_unset - Frees all session variables (It is equal to using: $_SESSION = array(); for older deprecated code)
2. unset($_SESSION['Products']); - Unset only Products index in session variable. (Remember: You have to use like a function, not as you used)
3. session_destroy — Destroys all data registered to a session

To know the difference between using session_unset and session_destroy, read this SO answer. That helps.

Unset will destroy a particular session variable whereas session_destroy() will destroy all the session data for that user.

It really depends on your application as to which one you should use. Just keep the above in mind.

unset($_SESSION['name']); // will delete just the name data

session_destroy(); // will delete ALL data associated with that user.

What about

unset($_SESSION["products"])

instead of the

session_destroy()

There is only one session per user. So there is no way to destroy a "specific" session. What you can do is delete the contents of your session responsible for the display of the cart (as shown above).

Please check your code is not right

Your wrong code

 if ($curLocation='london') 

Correct

if ($curLocation=='london') 

session_unset just clears the $_SESSION variable. It’s equivalent to doing:

$_SESSION = array();

So this does only affect the local $_SESSION variable instance but not the session data in the session storage.

In contrast to that, session_destroy destroys the session data that is stored in the session storage (e.g. the session file in the file system).

Everything else remains unchanged.

Use unset() for all the session variables specific to either site 1 or 2.

unset($_SESSION['site1']);
//or
unset($_SESSION['site2']);

Just so that you know, session_is_registered is deprecated as of PHP version 5.3.0. See docs.

Try using wp_doing_ajax() like so:

function unset_filter_session() {
    if ( ! wp_doing_ajax() ) {
        //Reset sessions on refresh page
        unset( $_SESSION['expenditure_filter'] );
    }
}

UPDATE

You can check the answer's revision for this update part..

UPDATE #2

Sorry, I didn't realize that you're loading a page fragment (#content-area) using the jQuery.load() method. Or that you're not using the admin-ajax.php to handle the AJAX request.

So if you're not using the wp_ajax_{action} or wp_ajax_nopriv_{action} action, or with the way you do the AJAX request, you can check whether the X-Requested-With header is set and that its value is XMLHttpRequest, and if so, you can cancel the session unsetting, like so:

function unset_filter_session() {
    if ( empty( $_SERVER['HTTP_X_REQUESTED_WITH'] ) ||
        'XMLHttpRequest' !== $_SERVER['HTTP_X_REQUESTED_WITH'] ) {
        //Reset sessions on refresh page, if not doing AJAX request
        unset( $_SESSION['expenditure_filter'] );
    }
}

That should work because jQuery always sets that header, unless you explicitly remove it — or change its value.

See the headers option on the jQuery.ajax() reference:

The header X-Requested-With: XMLHttpRequest is always added, but its default XMLHttpRequest value can be changed

Note: The header name is X-Requested-With, but in the superglobal $_SERVER, its key is HTTP_X_REQUESTED_WITH. I.e. Don't use $_SERVER['X-Requested-With'].

UPDATE #3

As suggested by Lawrence Johnson, you (ahem, we) should use filter_input():

function unset_filter_session() {
    if ( 'XMLHttpRequest' !== filter_input( INPUT_SERVER, 'HTTP_X_REQUESTED_WITH' ) ) {
        //Reset sessions on refresh page, if not doing AJAX request
        unset( $_SESSION['expenditure_filter'] );
    }
}

(but I kept the previous code for reference)