The issue you are facing has to do with encoding. Since these are considered special characters, you need to add some encoding details when converting to hex.
Each of these outputs exactly what you were looking for when I run them:
echo bin2hex(iconv('UTF-8', 'ISO-10646-UCS-2', '人')) . PHP_EOL;
//Outputs 4eba
echo bin2hex(iconv('UTF-8', 'UNICODE-1-1', '人')) . PHP_EOL;
//Outputs 4eba
echo bin2hex(iconv('UTF-8', 'UTF-16BE', '人')) . PHP_EOL;
//Outputs 4eba
Pick whichever one you fancy.
If you want to convert back:
echo iconv('UTF-16BE', 'UTF-8', hex2bin('4eba')) . PHP_EOL;
//outputs 人