Specific HTML strip Method in Php

<?php
function strip_only($str, $tags) {
    if(!is_array($tags)) {
        $tags = (strpos($str, '>') !== false ? explode('>', str_replace('<', '', $tags)) : array($tags));
        if(end($tags) == '') array_pop($tags);
    }
    foreach($tags as $tag) $str = preg_replace('#</?'.$tag.'[^>]*>#is', '', $str);
    return $str;
}

$str = '<p style="text-align:center">Paragraph</p><strong>Bold</strong><br/><span style="color:red">Red</span><h1>Header</h1>';

echo strip_only($str, array('p', 'h1'));
echo strip_only($str, '<p><h1>');
?>

Output:
Paragraph<strong>Bold</strong><br/><span style="color:red">Red</span>Header