Pass variable to PHP file which creates XML

The code below selects all records from the specified table, and outputs the results to XML.  I use the following constructor in HTML to create a dataset from these records:

var myDataset = new Spry.Data.XMLDataSet("xml/myTable.php", "root/record");

My question is, how can I pass a variable from the constructor line into the PHP file which will act as a filter?  What if I only want a subset of records from the underlying table?  Thanks for your help.

<?php
$hostname_conn = "localhost";
$database_conn = "myDB";
$username_conn = "root";
$password_conn = "root";
$conn = mysql_pconnect($hostname_conn, $username_conn, $password_conn) or trigger_error(mysql_error(),E_USER_ERROR); 
?>
<?php
mysql_select_db($database_conn, $conn);
//$query_rsAll = "SELECT * FROM myTable";
$rsAll = mysql_query($query_rsAll, $conn) or die(mysql_error());
$row_rsAll = mysql_fetch_assoc($rsAll);
$totalRows_rsAll = mysql_num_rows($rsAll);
 
// Send the headers
header('Content-type: text/xml');
header('Pragma: public');        
header('Cache-control: private');
header('Expires: -1');
?>
<?php echo('<?xml version="1.0" encoding="utf-8"?>'); ?>
<root>
  <?php if ($totalRows_rsAll > 0) { // Show if recordset not empty ?>
  <?php do { ?>
        <record>
                <?php foreach ($row_rsAll as $column=>$value) { ?>
                <<?php echo $column; ?>><![CDATA[<?php echo $row_rsAll[$column]; ?>]]></<?php echo $column; ?>>
                <?php } ?>
        </record>
    <?php } while ($row_rsAll = mysql_fetch_assoc($rsAll)); ?>
        <?php } // Show if recordset not empty ?>
</root>
<?php
mysql_free_result($rsAll);
?>

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