Is there an xml_encode() like json_encode() in PHP?

You can define your own xml_encode() function such as this the one from http://darklaunch.com/2009/05/23/php-xml-encode-using-domdocument-convert-array-to-xml-json-encode

function xml_encode($mixed, $domElement=null, $DOMDocument=null) {
    if (is_null($DOMDocument)) {
        $DOMDocument =new DOMDocument;
        $DOMDocument->formatOutput = true;
        xml_encode($mixed, $DOMDocument, $DOMDocument);
        echo $DOMDocument->saveXML();
    }
    else {
        // To cope with embedded objects 
        if (is_object($mixed)) {
          $mixed = get_object_vars($mixed);
        }
        if (is_array($mixed)) {
            foreach ($mixed as $index => $mixedElement) {
                if (is_int($index)) {
                    if ($index === 0) {
                        $node = $domElement;
                    }
                    else {
                        $node = $DOMDocument->createElement($domElement->tagName);
                        $domElement->parentNode->appendChild($node);
                    }
                }
                else {
                    $plural = $DOMDocument->createElement($index);
                    $domElement->appendChild($plural);
                    $node = $plural;
                    if (!(rtrim($index, 's') === $index)) {
                        $singular = $DOMDocument->createElement(rtrim($index, 's'));
                        $plural->appendChild($singular);
                        $node = $singular;
                    }
                }

                xml_encode($mixedElement, $node, $DOMDocument);
            }
        }
        else {
            $mixed = is_bool($mixed) ? ($mixed ? 'true' : 'false') : $mixed;
            $domElement->appendChild($DOMDocument->createTextNode($mixed));
        }
    }
}

JSON can express php arrays, integers, strings, etc. natively. XML has no such concepts - just elements, attributes, and text. If you want to transfer an object verbatim, use JSON. If you want to implement a complex API, use XML, for example the php DOM interface.