One not so well known feature of PHP is extract(), a function that unpacks an associative array into the local namespace. This probably exists for the autoglobal abormination but is very useful for templating:
function render_template($template_name, $context, $as_string=false)
{
extract($context);
if ($as_string)
ob_start();
include TEMPLATE_DIR . '/' . $template_name;
if ($as_string)
return ob_get_clean();
}
Now you can use render_template('index.html', array('foo' => 'bar')) and only $foo with the value "bar" appears in the template.