<?
    $content = "some text [again < dummy1>< /dummy1> some te < dummy2>< /dummy2>< /dummy3>xt] keep theses < needkeeped>xxx< needkeeped>";
    echo preg_replace_callback('/\[[^\]]+\]/', function($matches){
        return preg_replace('/<[^>]*>/', '', $matches[0]);
    }, $content);
    

?>