In your ajax call, alongwith the url you can send a parameter, say- action:
....
url: "WebService.php?action=checkLogin",
....
Then in PHP, just get this variable and call the desired function-
if(isset($_GET['type'])){
if($_GET['type'] == "checkLogin"){
$ws = new WebService;
echo $ws->CheckUserLogin();
// this will call the desired function and the result will be obtained by the ajax
}
}
Similarly you can use the same concept to call different functions at different points.