PHP allow template to use HTML only

I have a place where i have to include a HTML template. The HTML is written by employees only but i dont wanna be an idiot and include it without masking|checks :)

It should allow HTML tags only without any attributes.

So no <a href... links and no <div style=... divs or w/e.

My test script:

$string
    = '
<p>
    <strong>Foo</strong>
</p> 
<p>Bar</p> 
<p>
    <strong>Baz</strong>&nbsp;Mmmpf
</p>
<ul>
    <li>someting</li>
    <li>someting more</li>
    <li>even more</li>
</ul> 
<p>
    <strong>Foo</strong>
</p> 

<i>Foo <u>Bar</u></i>Baz

<!-- xss -->
<script>alert(1)</script>
<p onmouseover="alert(1)"></p>
<!-- ... -->
';


$htmlWhitelist = [
    'u',
    'i',
    'p',
    'strong',
    'ul',
    'li',
];
// replace allowed tags with placeholders 
// that not get changed by htmlspecialchars()
foreach ($htmlWhitelist as $tag) {
    $string = str_replace(
        ["<{$tag}>", "</{$tag}>"],
        ["{OPEN}{$tag}{OPEN}", "{CLOSE}{$tag}{CLOSE}"],
        $string
    );
}
// htmlspecialchars() on everything
$string = htmlspecialchars($string);
// put back the allowed tags
foreach ($htmlWhitelist as $tag) {
    $string = str_replace(
        ["{OPEN}{$tag}{OPEN}", "{CLOSE}{$tag}{CLOSE}"],
        ["<{$tag}>", "</{$tag}>"],
        $string
    );
}

I cannot imagine anything could go wrong with this but would like to ask you guys if i missed someting.