Merge XML files in PHP

I have 2 files 1.xml and 2.xml both having similar structure and I would like to have one. I tried many solutions but I had errors only - frankly speaking I have no idea how those scripts worked.

1.xml:

<res>
    <items total="180">
        <item>
            <id>1</id>
            <title>Title 1</title>
            <author>Author 1</author>
        </item>
        ...
    </items>
</res> 

2.xml:

<res>
    <items total="123">
        <item>
            <id>190</id>
            <title>Title 190</title>
            <author>Author 190</author>
        </item>
        ...
    </items>
</res> 

I would like to create a new file merged.xml with the following structure

<res>
    <items total="303">
        <item>
            <id>1</id>
            <title>Title 1</title>
            <author>Author 1</author>
        </item>
        ...  //items from 1.xml
        <item>
            <id>190</id>
            <title>Title 190</title>
            <author>Author 190</author>
        </item>
        ... //items from 2.xml
    </items>
</res> 

How should I do that? Can you explain me the way to do it? How can I do it with more files? Thanks

Edit

What I tried?

<?php
function mergeXML(&$base, $add)
{
    if ( $add->count() != 0 )
    $new = $base->addChild($add->getName());
    else
        $new = $base->addChild($add->getName(),