You can check the page name based on the value of: $_SERVER['PHP_SELF']
So you might use:
<a href="about.php" class="<?php echo(($_SERVER['PHP_SELF']=="about.php")?"active":""); ?>">about us</a>
Write the value of: <?php echo($_SERVER['PHP_SELF']); ?> to each page to get the correct value for comparison (I'm not sure if it includes a slash or not in the front)