它就来了（假设使用UTF-8，但是更改起来很简单）：

function encode($str) {
    $str = mb_convert_encoding($str , 'UTF-32', 'UTF-8'); //big endian
    $split = str_split($str, 4);

    $res = "";
    foreach ($split as $c) {
        $cur = 0;
        for ($i = 0; $i < 4; $i++) {
            $cur |= ord($c[$i]) << (8*(3 - $i));
        }
        $res .= "&#" . $cur . ";";
    }
    return $res;
}

编辑 推荐的替代使用unpack：

function encode2($str) {
    $str = mb_convert_encoding($str , 'UTF-32', 'UTF-8');
    $t = unpack("N*", $str);
    $t = array_map(function($n) { return "&#$n;"; }, $t);
    return implode("", $t);
}