I am building a new site which uses php includes in the home page. In the other pages which are in some other subfolders, I want to use the same navigation and header includes. But if I use the same includes in the index.php and other files in other folders, the images, animation etc. will not work.
For eg., the image topleft is the logo. It is in the image sfolder. So in the logo.php file the code will be <img src="images/topleft.gif"> which will work for the home page well. But in other pages, the code will simply not worl because hte folder images is not in the folder where the files are.
Could someone please help me out with this. I think I had read something about $_server['http_host'] a long time age which was related to this.

just use a forward slash before the path to the images, like this:
<img src="/images/topleft.gif">

This will instruct the browser to start at the root of your site (/) and look for the images folder. Without the forward slash, your browser will always start at the current subdirectory.

You could use absolute urls

I wouldn't use absolute urls though ... you are better off with the "/" trust me ... I just moved two huge sites ... one used absolute urls and the other used "/". I had to go into every page that had an absolute url and change it. All my sites now use the "/" to pull from the root.

Good point

Have you ever installed a script which asks you for the URL of the site? The URL is stored in a configuration file for the purpose of concatenating to partial URLs to form absolutes.

Create a config.php file and include a var called $root with the value 'http://www.myDomain.com/'

Include the config file with your pages and concatenate URL's to give absolutes; e.g. $root.'images/logo.gif';