How can I tell where the script is being called from?

[ablaye]

Hello,

Here is what I want to do. I want to have a variable that tells me whether the page that is loaded is from the root directory (i.e. www.domain.com/xyz.php) or whether it is being called from the blog directory (i.e. www.domain.com/blog/xyz.php).
Based on what pages are getting loaded, i want to be able to change things in the common header.
How do I do this?

Thanks