On that index.php, put
<?php echo $_SERVER['HTTP_HOST']; ?>
That will print out the name of the domain you are on.
You can then do something like:
<img src='/images/<?php echo $_SERVER['HTTP_HOST']; ?>.jpg'>
for instance to pull a custom logo for that site, or grab <?php echo $_SERVER['HTTP_HOST']; ?>.css to load a different template, or whatever.
That's a really simplified version of the idea but you should be able to elaborate from there.