Running php code in WordPress pages

Sorry if this is a daft question, but I could do with some guidance on the following if possible.

I had a website (not WordPress) that consisted of a number of php files, one for each page plus a header.php, footer.php, navigation.php etc.

In each page file eg contact_us.php, I would add the following to the top of the php file, after the DOCTYPE and before the HTML tag:

<?php $thisPage=”Contact us”; ?>

In navigation.php, I then had a series of IF statements like this:

<?php if ($thisPage==”Contact us”)
echo ” class=\”sub_menu selected\””; else echo ” class=\”sub_menu\””; ?>>

This was to allow the navigation link for the page I was on to be highlighted.

I’ve now had my website moved into WordPress, and each original page has been added into WorPress as a ‘page’, according to the admin panel.
But, each page no longer has the ‘<?php $thisPage=”Contact us”; ?>’ code in it, so the navigation highlight check can’t pick up which page I’m on. I tried installing Exec PHP and adding ‘<?php $thisPage=”Contact us”; ?>’ to the top of each page, but it doesn’t seem to work.

Can anyone tell me what I need to do so that my navigation.php can pick up the $thisPage variable and handle it as it did pre-WordPress?

Many thanks for any help you can give.