PHP - Creating An Xml Document That Does Not Print To The Screen

I have managed to retrieve an sql query and through php create an xml document, my next aim is to get this code into actionscript so that I can use the data in Flash. One problem I am experiencing is that in creating the xml document the data within xml nodes are echoed to the screen, I do not want them to be echoed to the screen just picked up by actionscript. Is there a way to created an xml document using php that does not print to the screen?

<?php

include("php/dbconnect.php"); //connects to the database


//sql_query
$table_id = 'booking'; 
$query = "SELECT * FROM $table_id WHERE (booking.bookingDate = curDate()) AND roomID = 1 ORDER BY startTime";
$room1 = mysql_query($query);


//create a new DOM document
$doc = new DOMDocument('1.0');

//create root element
$root = $doc->createElement('root');
$root = $doc->appendChild($root);

//process one row at at time
while($row = mysql_fetch_assoc($room1)){

//add node for each row
$occ = $doc->createElement($table_id);
$occ = $doc->appendChild($occ);

//add achild for each field
foreach ($row as $fieldname => $fieldvalue){

$child = $doc->createElement($fieldname);
     $child = $occ->appendChild($child);


$value = $doc->createTextNode($fieldvalue);
     $value = $child->appendChild($value);
}
}

$xml_string = $doc->saveXML();

echo $xml_string;

mysql_free_result($room1);
?>