Change class for active menu item with php - php

Question

I am trying to change the class of an active element in my menu with php, but I am having trouble when using "?side=testpage" as URL
I'm typing the href in php to get the pages opened in the correct div according to my index where this code is included.
The whole code is:
<?php
function echo_Active_Tab ($requestUri){
$current_file_name = basename($_SERVER['REQUEST_URI'], ".php");
if ($current_file_name == $requestUri)
echo 'class="active"';
}
?>
<div class="navbar">
<div class="navbar-inner">
<a class="brand"<?php echo "href='?side=mainpage'>Økonomiprogrammet</a>"; ?>
<ul class="nav">
<li <?=echo_Active_Tab("mainpage")?>><?php echo "<a href='?side=mainpage.php'>Startside</a>"; ?></li>
<li <?=echo_Active_Tab("overview")?>><?php echo "<a href='?side=overview'>Oversikten</a>"; ?> </li>
<li <?=echo_Active_Tab("invoice")?>><?php echo "<a href='?side=invoice'>Fakturaer</a>"; ?></li>
<li <?=echo_Active_Tab("expences")?>><?php echo "<a href='?side=expences'>Utgifter</a>"; ?></li>
</ul>
<form class="navbar-form navbar-right" role="search">
<div class="form-group">
<input type="text" class="form-control" placeholder="Search">
<button type="submit" class="btn btn-default">Søk</button>
</div>
</form>
</div>
</div>
Does anyone spot the error in my code?
EDIT: Don't mind the wrong function name, fixed and edited. The code doesn't do as I'd hoped.