Something

<?php
class cart
{
	function showItem($item, $foo)
	{
		// Do something with $foo
		echo $item.'<br />'.$foo;
	}
}

class extendedCart extends cart
{
	function extendedShowItem($item)
	{
		parent::showItem($item, 'bar');
	}
}

$instance = new extendedCart();
$instance->extendedShowItem('foo');