While http_build_query can also be used to encode most classes, into a query string, SimpleXML Elements with <![CDATA[]]> values are picked up as empty arrays, and therefore aren't included naturally.
<?php
$xml = simplexml_load_string( '<wrapper><key><![CDATA[value]]></key><key2>value2</key2></wrapper>' );
var_dump( $xml, http_build_query( $xml ) );
/* Outputs:
object(SimpleXMLElement)#1 (2) {
["key"]=>
object(SimpleXMLElement)#2 (0) {
}
["key2"]=>
string(6) "value2"
}
string(11) "key2=value2"
*/
?>