function utf8($num)
{
    if($num<=0x7F)       return chr($num);
    if($num<=0x7FF)      return chr(($num>>6)+192).chr(($num&63)+128);
    if($num<=0xFFFF)     return chr(($num>>12)+224).chr((($num>>6)&63)+128).chr(($num&63)+128);
    if($num<=0x1FFFFF)   return chr(($num>>18)+240).chr((($num>>12)&63)+128).chr((($num>>6)&63)+128).chr(($num&63)+128);
    return '';
}

function uniord($c)
{
    $ord0 = ord($c{0}); if ($ord0>=0   && $ord0<=127) return $ord0;
    $ord1 = ord($c{1}); if ($ord0>=192 && $ord0<=223) return ($ord0-192)*64 + ($ord1-128);
    $ord2 = ord($c{2}); if ($ord0>=224 && $ord0<=239) return ($ord0-224)*4096 + ($ord1-128)*64 + ($ord2-128);
    $ord3 = ord($c{3}); if ($ord0>=240 && $ord0<=247) return ($ord0-240)*262144 + ($ord1-128)*4096 + ($ord2-128)*64 + ($ord3-128);
    return false;
}

utf8() and uniord() try to mirror the chr() and ord() functions on php:

echo utf8(0x6211)."\n";
echo uniord(utf8(0x6211))."\n";
echo "U+".dechex(uniord(utf8(0x6211)))."\n";

//In your case:
$wo='U+6211';
$hao='U+597D';
echo utf8(hexdec(str_replace("U+","", $wo)))."\n";
echo utf8(hexdec(str_replace("U+","", $hao)))."\n";

output:

我
25105
U+6211
我
好

I encountered the same problem recently, so was glad to see this question. Doing some tests, I found the following code works:

$replacedString = preg_replace("/\\\\u([0-9abcdef]{4})/", "&#xoriginal_string);
//$unicodeString    = mb_convert_encoding($replacedString, 'UTF-8', 'HTML-ENTITIES'); 

The only thing I changed is that I commented out the 2nd line of code. Webpage, however, must be set to display UTF-8.

Enjoy!

With PHP 7, there is a new IntlChar::ord() to find the Unicode Code Point from a given UTF-8 character:

var_dump(sprintf('U+%04X', IntlChar::ord('ß')));

# Outputs: string(6) "U+00DF"