用php安全过滤html?

function validCleanHtml( $unclosedString )
{
    preg_match_all( "/<([^\/]\w*)>/", $closedString = $unclosedString, $tags );
    for ( $i = count( $tags[1] ) - 1; $i >= 0; $i-- )
    {
        $tag = $tags[1][$i];
        if ( substr_count( $closedString, "</$tag>" ) < substr_count( $closedString, "<$tag>" ) )
            $closedString .= "</$tag>";
    }
    $validTags = "<em><strong>";
    $validClosedString = strip_tags( $closedString, $validTags );
    return $validClosedString;

}

好的,我想要的是启用2个html,em和strong,这只是从xss安全吗?如果没有,我们怎么能保证?