function unescape($str)
{
    $ret = '';
    $len = strlen($str);
    for ($i = 0; $i < $len; $i ++)
    {
        if ($str[$i] == '%' && $str[$i + 1] == 'u')
        {
            $val = hexdec(substr($str, $i + 2, 4));
            if ($val < 0x7f)
                $ret .= chr($val);
            else
                if ($val < 0x800)
                    $ret .= chr(0xc0 | ($val >> 6)) .
                     chr(0x80 | ($val & 0x3f));
                else
                    $ret .= chr(0xe0 | ($val >> 12)) .
                     chr(0x80 | (($val >> 6) & 0x3f)) .
                     chr(0x80 | ($val & 0x3f));
            $i += 5;
        } else
            if ($str[$i] == '%')
            {
                $ret .= urldecode(substr($str, $i, 3));
                $i += 2;
            } else
                $ret .= $str[$i];
    }
    return $ret;
}

echo unescape('A%u7ec4'). PHP_EOL;

function replace_unicode_escape_sequence($match) {
    return mb_convert_encoding(pack('H*', $match[1]), 'UTF-8', 'UCS-2BE');
}
function unicode_decode($str) {
    return preg_replace_callback('/[\\\\|%]u([0-9a-f]{4})/i', 'replace_unicode_escape_sequence', $str);
}
echo unicode_decode('A%u7ec4') . PHP_EOL;
echo unicode_decode('A\u7ec4') . PHP_EOL;

echo unicode_decode(utf8_decode('A\u7ec4'));