转换unicode十进制内码为utf-8编码

function uni($str)
{
    $ret = '';
    for ($i = 0; $i < mb_strlen($str, 'utf-8'); $i = $i + 1) {
        $ret .= "&#" . uniord(mb_substr($str, $i, 1, 'utf-8')) . ";";
    }
    return $ret;
}

function uniord($u)
{
    $c = unpack("N", mb_convert_encoding($u, 'UCS-4BE', 'UTF-8'));
    return $c[1];
}

function unichr($u)
{
    return mb_convert_encoding(pack("N", $u), mb_internal_encoding(), 'UCS-4BE');
}

function u2utf8($c)
{
    $str = "";
    if ($c < 0x80) {
        $str .= chr($c);
    } else if ($c < 0x800) {
        $str .= chr(0xC0 | $c >> 6);
        $str .= chr(0x80 | $c & 0x3F);
    } else if ($c < 0x10000) {
        $str .= chr(0xE0 | $c >> 12);
        $str .= chr(0x80 | $c >> 6 & 0x3F);
        $str .= chr(0x80 | $c & 0x3F);
    } else if ($c < 0x200000) {
        $str .= chr(0xF0 | $c >> 18);
        $str .= chr(0x80 | $c >> 12 & 0x3F);
        $str .= chr(0x80 | $c >> 6 & 0x3F);
        $str .= chr(0x80 | $c & 0x3F);
    }
    return $str;
}

$source = '好好学习,天天向上';
$source = uni($source);
var_dump($source);
// string(69) "&#22909;&#22909;&#23398;&#20064;&#44;&#22825;&#22825;&#21521;&#19978;"
 //
preg_match_all("/&#([0-9]+);/", $source, $regs);
var_dump($regs);
foreach ($regs[1] as $v) {
    $source = str_replace("&#$v;", u2utf8($v), $source);
}
var_dump($source);
// string(25) "好好学习,天天向上" //