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;
}

编辑推荐替代使用unpack：@H_502_4@

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