If you are submitting a form and handling the request using PHP, you will not be able to access the DOM in PHP (client vs server). If you can pull out the bits that you need using javascript, you can set the values on hidden form elements and submit.
<?php
// print out the value when the post is submitted
if (isset($_POST["extraInput"])) {
echo "hidden input is: " + $_POST["extraInput"];
}
?>
<html>
<head>
<script type="text/javascript">
function doSubmit () {
var extraValue = document.getElementById("extra").innerHTML;
var form = document.forms["myForm"];
form.elements["extraInput"].value = extraValue;
form.submit();
}
</script>
</head>
<div id="extra">Hello world</div>
<body>
<form id="myForm" action="" method="post">
<input type="hidden" name="extraInput" />
<input type="text" name="textInput" />
<button onclick="javascript:doSubmit()">Submit</button>
</form>
</body>
</html>