/**
 * $str 原始中文字符串
 * $encoding 原始字符串的编码，默认GBK
 * $prefix 编码后的前缀，默认"&#"
 * $postfix 编码后的后缀，默认";"
 */
function unicode_encode($str, $encoding = 'GBK', $prefix = '&#', $postfix = ';') {
    $str = iconv($encoding, 'UCS-2', $str);
    $arrstr = str_split($str, 2);
    $unistr = '';
    for($i = 0, $len = count($arrstr); $i < $len; $i++) {
        $dec = hexdec(bin2hex($arrstr[$i]));
        $unistr .= $prefix . $dec . $postfix;
    }
    return $unistr;
}

/**
 * $str Unicode编码后的字符串
 * $decoding 原始字符串的编码，默认GBK
 * $prefix 编码字符串的前缀，默认"&#"
 * $postfix 编码字符串的后缀，默认";"
 */
function unicode_decode($unistr, $encoding = 'GBK', $prefix = '&#', $postfix = ';') {
    $arruni = explode($prefix, $unistr);
    $unistr = '';
    for($i = 1, $len = count($arruni); $i < $len; $i++) {
        if (strlen($postfix) > 0) {
            $arruni[$i] = substr($arruni[$i], 0, strlen($arruni[$i]) - strlen($postfix));
        }
        $temp = intval($arruni[$i]);
        $unistr .= ($temp < 256) ? chr(0) . chr($temp) : chr($temp / 256) . chr($temp % 256);
    }
    return iconv('UCS-2', $encoding, $unistr);
}

//GBK字符串测试
$str = '哈哈';
echo $str.'
';

$unistr = unicode_encode($str);
echo $unistr.'
'; // <b>哈哈</b>

$str2 = unicode_decode($unistr);
echo $str2.'
'; //哈哈

//UTF-8字符串测试
$utf8_str = iconv('GBK', 'UTF-8', $str);
echo $utf8_str.'
'; // 鍝堝搱 注：UTF在GBK下显示的乱码！可切换浏览器的编码测试

$utf8_unistr = unicode_encode($utf8_str, 'UTF-8');
echo $utf8_unistr.'
'; // <b>哈哈</b>

$utf8_str2 = unicode_decode($utf8_unistr, 'UTF-8');
echo $utf8_str2.'
'; // 鍝堝搱

//其它后缀、前缀测试
$prefix_unistr = unicode_encode($str, 'GBK', "\u", '');
echo $prefix_unistr.'<br />'; // u60u98u62u21704u21704u60u47u98u62

$profix_unistr2 = unicode_decode($prefix_unistr, 'GBK', "\u", '');
echo $profix_unistr2.'<br />'; //<b>哈哈</b>