How will you add the query string in url using l() function?

If you want to use the l() function and provide the destination:

('Login first','user/login', array(
'query' => array(
'destination' => 'node/'. $node->nid
)
)
);
?>

The above code will provide you with an anchor tag with the href pointed to user/login?destination=node/NID

This is only useful if you are linking to a page that consist of a form, and you want the user to be redirected to another page after submitting the form.

Ref: http://api.drupal.org/api/drupal/includes--common.inc/function/l/6