Converting a number to an UnsignedByte in PHP

Question

I wish to pass jSON data as POST to a RESTful web service using PHP.

The json data to be passed one field is said to be an UnsignedByte

{"merchantNumber": "214", "meterNumber": "371315479968"}

the value meterNumber is the one that has to be UnsignedByte

I need help in coverting the value to UnsignedByte like so

    function convertToUnsignedByte( $string ){     
         $result = '';// DO SOMETHING WITH $string;
         return $result;
        }

        $string  = "14656575668";

        $data = json_encode(array(
                              "merchantNumber": $merchantNumber, 
                              "meterNumber": convertToUnsignedByte($string)
                               )
                           );