Do defined variables exist at application scope, or session scope?

Hi All, In the index.php file of my application I define several variables, including such things as the base path of the app, and the theme path etc. Since I use 'define()' to do this, are these variables available to my app regardless of where a particular user enters the app? So, in starting the app, I define these variables by visiting index.php. But if another user gets sent a url to, for example, the help page, when they visit it will those variables be available, or will I need to explicitly check on each to make sure the variables are defined, because the user entered at a different entry point than the 'normal' one? Note: I will probably do an explicit check anyway, since this seems more robust, but I ask to better understand how define works. Many thanks, M is for Murray