XML removing closing/opening tags

I have a problem with a piece of XML that I want to parse with PHP. Here is the example I have:

<tags>
    <content>content</content>
    <amplifications>
        <tag>content 1</tag>
    </amplifications>
    <amplifications>
        <tag>content 2</tag>
        <tag>content 3</tag>
        <tag>content 4</tag>
        <tag>content 5</tag>
    </amplifications>
</tags>

Where I want to remove

</amplifications>
<amplifications>

I've tried using preg_replace, but it seems that I cannot figure it out because those tags are indented differently and there are whitespaces.