I am Parsing a HTML document using DOMDocument Class in PHP, i wanted to get the nodeValue of a div element, but it is giving me null,

<div id="summary">
   Hi, my name is <span>ABC</span>
   <br/> 
   address is here at stackoverflow...
   <span>....
   ....
<div>

want to get the value inside the div, and the code i wrote wass

$div_node=$dom->getElementById("summary");
$node_value=$div_node->nodeValue;

but it is giving me a null value, please help.