.example1 {
display: block
}

.example1 {
display: none
}

$page= substr($url, strrpos($url, '/') + 1);

<?php
if($page = "page123")
   echo css file
else
   //Echo another css file
?>

var url = window.location.href;
var param = url.splice('?')[1]; // Get your class "ex example1"

$('body').addClass(param);

var urlparam = "?" + param;

body.example1 div.foo {
   display: none
}

body.example2 div.foo {
   display: block
}