If you're using prepared statements, then you don't need to sanitise it. If you're inserting the values into the query itself, then you're using PDO wrong (but you would need to sanitise it).
Apache, when misconfigured, can use a lot of memory. By default, it is configured for large servers - so it will use a lot of memory, but will be able to handle a lot of users. If you configure it properly, it's fine.
I added [html] and [code] tags to both of your posts - please use them in the future.
Adding it to the window object will not mean that you can access it across pages, you want to set a cookie or something.
I'm not really sure what you're asking - could you give us a context and example...