I have my code removing the starting tags, but now I want to replace the ending
$content = preg_replace('#<p(.*?)>(.*?)</p>#is', '$2<br/>', $content);
use str_replace instead of preg_replace, so:
$content = '<p>This is a new content for missing slash</p>';
$newcontent = preg_replace("/<p[^>]*?>/", "", $content);
$newcontent = str_replace("</p>", "<br />", $newcontent);