<?php
function makeNav($pageArray){
   $output = "<ul id=\"nav\">\n";
   foreach($pageArray as $url => $label){
      $classAttribute = basename($_SERVER['PHP_SELF']) == $url ? ' class="active"' : '';
      $output .= " <li><a href=\"$url\"$classAttribute>$label</a></li>\n";
   }
   return $output . "</ul>\n";
}

// usage follows
// define an array of pages and labels...
$pages = array('index.php' => 'HOME',
               'about.php' => 'ABOUT',
               'projects.php' => 'PROJECTS',
               'irrigation.php' => 'IRRIGATION',
               'news.php' => 'NEWS',
               'contact.php' => 'CONTACT');

// echo the output where you need it
echo makeNav($pages);