Restrict Html Tags which User Can Input (PHP)

I tried to implement very simple html edit text area which has available tags user can input are restricted.
So I needed to implement a validator which detects tags not allowed to use.

The proper (but a bit heavy) implementation approach is using Tidy.
It can validate entire html and also fix and clean up html source!

However in my case using tidy is a bit overkill solution.
Instead of using tidy, I decided to use strip_tags function.
The disadvantage is that the function does not validate html syntax. e.g. inaccurate than using tidy.- "strip_tags function does not actually validate the html, partial or broken tags can result in the removal of more text/data than expected." as the official PHP document says.

Okie, as long as we understand the disadvantage, we can use this function.
Let's show you the code.

function validateOnlyAllowedTags($html, $tags)
{
   $stripped = strip_tags($html, $tags);

   // if no tags are stripped, the length of html contents should not be changed
   return strlen($stripped) == strlen($html);
}

The function usage is below:

$html = '<h2>Title</h2>'
$result = validateOnlyAllowedTags($html, 'h2');
echo($result); // should be true!

$result = validateOnlyAllowedTags($html, 'h3');
echo($result); // should be false!

Hope this post helps :D