<ul>
  <li>Backup
    <ul>
      <li>Tape
        <ul>
          <li>LTO</li>
          <li>RTO</li>
          ...
        </ul>
      </li>
    </ul>
  </li>
  <li>Motherboard
    <ul>
       ...
    </ul>
  </li>
</ul>

<?php

class ProductList
{
    private $m_aList;
   
    public function __construct($aProducts)
    {
        $this->m_aList = array();
       
        foreach ($aProducts as $szProductTree)
        {
            $this->parseProductTree($szProductTree);
        }            
    }
   
    public function parseProductTree($szProduct)
    {
        // First of all split the string into an array
        $aParts = split('/', $szProduct);
       
        // Get a reference to root array
        $aList = &$this->m_aList;
       
        $iLastIndex = count($aParts) - 1;

        foreach ($aParts as $iIndex => $szLabel)
        {
            // This is the last node in the list, we need to handle this
            // differently because it will not contain any child nodes
            if ($iIndex == $iLastIndex)
            {
                if (key_exists($szLabel, $aList))
                {
                    break; 
                }
               
                $aList[] = $szLabel;
                break;
            }

            // Check if node exists in the current array (specified by $aList), 
            // if it doesn't exist, we add it to the array
            if (!key_exists($szLabel, $aList))
            {
                $aList[$szLabel] = array();
            }
           
            // Update the array reference to point to the array we just added
            // The next node in the foreach loop will be added to this array
            $aList = &$aList[$szLabel];
        }      
    }
   
    public function getList()
    {
        return $this->m_aList;
    }
}

class ProductList_RenderHtml
{
    public function render($aList)
    {
        $szHtml .= "<ul>\n";
       
        foreach ($aList as $mNodeName => $mNodeValue)
        {
            $szHtml .= $this->renderList($mNodeName, $mNodeValue);
        }
       
        $szHtml .= "</ul>\n";
        return $szHtml;
    }
   
    public function renderList($szListLabel, $aList)
    {
        $szHtml .= "<li>" . $szListLabel . "\n";
        $szHtml .= "<ul>\n";
       
        foreach ($aList as $mNodeName => $mNodeValue)
        {
            // If the node value is an array we render it as a new list
            if (is_array($mNodeValue))
            {
                $szHtml .= $this->renderList($mNodeName, $mNodeValue);
            }
            // If its not an array we know it's list item
            else
            {
                $szHtml .= "<li>$mNodeValue</li>\n";
            }
        }
       
        $szHtml .= "</ul>\n";
        $szHtml .= "</li>\n";
        return $szHtml;
    }
}

$aProducts = array(
    'Backup/Tape/LTO',
    'Backup/Tape/RDX',
    'Backup/Tape/Cleaner',
    'Motherboard/Intel',
    'Motherboard/AMD'
);

$pProductList = new ProductList($aProducts);
$pRender = new ProductList_RenderHtml();

echo $pRender->render($pProductList->getList());

?>

$a = 1;
$b =& $a;
$b = 2;

$aArray = array('Apple', 'Orange', 'Kiwi', 'Pear');
$pItem =& $aArray[2];
$pItem = 'Pitaya';