Symfony Serialization组件 - 使用元素和属性序列化数组

I trying to serialize an array with symfony's serialize component. I follow this manual

How to create following xml document:

<foo tag="bar">baz</foo>

My array to serialize:

['foo' => [@tag => 'bar', 'baz']]

but it creates folowing:

<foo tag="bar">
    <item key="0">baz</item>
</foo>

Create array in this way:

['foo' => ['@tag' => 'bar', '#' => 'baz']]