use php variables in simple xml

We are just learning vxml and are trying to use $_REQUEST to get a variable from the url and place it in a dom doc to create a simple xml/vxml output -

We have tried the following code but get a parse error -
Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE, expecting T_STRING or T_VARIABLE or T_NUM_STRING in xxxxxxxxxxxxxx.php on line 10 -

What are we missing -
Thanks in advance for the help

<?php
$doc = new DomDocument('1.0');
 $doc ="<?xml version=\"1.0\" encoding=\"utf-8\"?>\n";
 $doc .="<vxml version = \"2.1\" >\n";
$doc .="<form id=\"mainDialog\">\n";
$doc .="<prompt> your caller id is <?php $_REQUEST['callerid'];?></prompt>\n";      
$doc .="</form>\n";
$doc .="</vxml>\n";
$sxe = new SimpleXMLElement($doc);
    echo $sxe->asXML();
?>