I probably already have this posted somewhere, I know I use it a ton but always have to look it up for the exact syntax. Here's how you can get the name of the current file (ie. if your file is "aboutus.php" this will echo "aboutus.php")
<?php
$currentFile = $_SERVER["PHP_SELF"];
$parts = Explode('/', $currentFile);
echo $parts[count($parts) - 1];
?>
I've seen variations on it, as I said, I always have to look it up for the exact syntax – but today I found that from here. Also here – which seems to list a bunch of other useful snippets.