Php navigation - changing background colour when active

[cherbert]

I have got a code working to generate a menu from a mysql table, but I wan't to know if it is possible to have the background colour change to a third option when active.

I have 1 background colour for 'off', one for mouseover, but can't figure out how to tell it what they current page is so that it will use a different css class value, as the page obviously reloads after the link is clicked. Is it possible (hope my explanation makes some sence!)

 

My code:

At the moment mainNav a: and mainNav a:hover work ok but what I want is a mainNav a:active to stay on after the page has loaded

<?php
//function to nest the menu tree
function nest_child(&$nodes, $current) {
	if (!isset($nodes) || count($nodes) < 1) {
		return;
	}
	foreach ($nodes as $id => $item) {
		if ($current['parentdocid'] == $id) {
			if (!isset($item['children'])) {
				$item['children'] = array();
			}
			array_push($item['children'], $current);
			$nodes[$id] = $item;
		} else {
			nest_child($item['children'], $current);
		}
	}
}

//functions to display the tree and expand as appropriate
function menu_display($nodes, $display_id) {
	if (!isset($display_id) || ($display_id == '')) {
		$display_id = 1;
	}
	$result = node_display(
		array(
			'doc_id'   => 1,
			'children' => $nodes
		),
		$display_id
	);
	return $result[1];
}
function node_display($node, $display_id) {
	global $ROOT;
	$found = false;
	$text = '';

	if (isset($node[children])) {
		foreach ($node[children] as $id => $child) {
			$text .= "<li class='mainNav'><a href='$ROOT/index.php?doc_id=$child[doc_id]'>$child[doc_name]</a>";
			$result = node_display($child, $display_id);
			if ($result[0]) {
				$found = true;
				$text .= "<ol>$result[1]</ol>";
			}
			if ($child[doc_id] == $display_id) {
				$found = true;
				if (isset($child[children])) {
					$text .= "<ol>";
					foreach ($child[children] as $id => $grandchild) {
						$text .= "<li class='mainNav'><a href='$ROOT/index.php?doc_id=$grandchild[doc_id]'>$grandchild[doc_name]</a>";
					}
					$text .= "</ol>";
				}
			}
			$text .= "</li>";
		}
	}
	if ((!$found) && ($display_id != 1)) {
		$text = '';
	}

	return array($found, $text);
}
?>

<ul class="menu">
<?php
	//retrieve all the menu elements
	$result = mysql_query('SELECT * FROM docs ORDER BY doc_order ASC');
	$menu = array();
	while ($row = mysql_fetch_assoc($result)) {
		$menu[$row['doc_id']] = $row;
	}

	//nest the elements
	foreach ($menu as $id => $item) {
		if ($item['parentdocid'] != 1) {
			nest_child($menu, $item);
			unset($menu[$id]);
		}
	}

	//display the tree
	print menu_display($menu, $doc_id);
?>
</ul>