/**
* Function: getElementById
*
* Returns the element with the given ID from
* <document>. The optional attr argument specifies
* the name of the ID attribute. Default is "id".
* The XPath expression used to find the element is
* //*[@attr='arg'] where attr is the name of the
* ID attribute and arg is the given id.
*
* Parameters:
*
* id - String that contains the ID.
* attr - Optional string for the attributename.
* Default is "id".
*/
function getElementById($id, $attr = "id")
{
$expr = "//*[@{$attr}='{$id}']";
return mxUtils::selectSingleNode($this->document, $expr);
}