Issue with stripping all tags except img and a in wordpress

I’m using WordPress and in my theme’s function.php I have a function to decode html entities and strip some tags:

function entities_and_strip($string, $allowable_tags) {
    $result = html_entity_decode($string)

    return strip_tags($result, $allowable_tags);
}

In single.php I call this funciton:

echo entities_and_strip(get_the_content(), "<img><a>");

The problem is it strips a tag as well and only allow img tag. If I remove img from allowable_tags then it works.

I noticed two things so far:

1. Passing <img> with any other tags (e.g. strip_tags($string, "<img><ul><li><a>");) will result in stripping all tags except img.
2. This only happens when I’m in WordPress environment. I wrote a PHP script with strip_tag inside and run it separately. It worked well.

What makes the problem? And how could I solve it?