<?php

use JMS\Serializer\SerializerBuilder;
use JMS\Serializer\Annotation as A;

// for annotations see
// http://jmsyst.com/libs/serializer/master/reference/annotations

/**
 * A\ExclusionPolicy("all")
 * @A\XmlRoot("data")
 */
class Data
{
    private $id = 1;
    
    private $name = 'John Doe';
    
    /**
     * @A\XmlKeyValuePairs
     */
    private $values = [
        'age' => 21,
        'haircolor' => 'brown',
    ];
    
    private $null = null;
}

$data = new Data();

$serializer = (new SerializerBuilder())
    ->build();

echo '<pre>' . htmlentities(
    $serializer->serialize($data, 'xml')
) . '</pre>';

<?xml version="1.0" encoding="UTF-8"?>
<data>
  <id>1</id>
  <name><![CDATA[John Doe]]></name>
  <values>
    <age>21</age>
    <haircolor><![CDATA[brown]]></haircolor>
  </values>
</data>

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PHP 7.2, Zend 3.0, Symfony 4.0, Laravel 5.6, PHPUnit 6.5, Doctrine 2.8