Voilà. Cela fonctionne à la fois dans WordPress et en dehors de WordPress.
<?php
$str = '     I am <strong>stronger</strong> and cooler every single day <aaaaa>.    ';
echo orbisius_html_util::strip_tags($str);
/**
 * Util HTML class
 * @author Svetoslav Marinov (SLAVI) | http://orbisius.com
 */
class orbisius_html_util {
    /**
     * Uses WP's wp_kses to clear some of the html tags but allow some attribs
     * usage: orbisius_html_util::strip_tags($str);
     * uses WordPress' wp_kses()
     * @param str $buffer string buffer
     * @return str cleaned up text
     */
    public static function strip_tags($buffer) {
        static $default_attribs = array(
            'id' => array(),
            'class' => array(),
            'title' => array(),
            'style' => array(),
            'data' => array(),
            'data-mce-id' => array(),
            'data-mce-style' => array(),
            'data-mce-bogus' => array(),
        );
        $allowed_tags = array(
            'div'           => $default_attribs,
            'span'          => $default_attribs,
            'p'             => $default_attribs,
            'a'             => array_merge( $default_attribs, array(
                'href' => array(),
                'target' => array('_blank', '_top'),
            ) ),
            'u'             =>  $default_attribs,
            'i'             =>  $default_attribs,
            'q'             =>  $default_attribs,
            'b'             =>  $default_attribs,
            'ul'            => $default_attribs,
            'ol'            => $default_attribs,
            'li'            => $default_attribs,
            'br'            => $default_attribs,
            'hr'            => $default_attribs,
            'strong'        => $default_attribs,
            'blockquote'    => $default_attribs,
            'del'           => $default_attribs,
            'strike'        => $default_attribs,
            'em'            => $default_attribs,
            'code'          => $default_attribs,
        );
        if (function_exists('wp_kses')) { // WP is here
            $buffer = wp_kses($buffer, $allowed_tags);
        } else {
            $tags = array();
            foreach (array_keys($allowed_tags) as $tag) {
                $tags[] = "<$tag>";
            }
            $buffer = strip_tags($buffer, join('', $tags));
        }
        $buffer = trim($buffer);
        return $buffer;
    }
}