Create a 48bit integer suitable for use with pack() in PHP

48bit_test1.php

<?php

$topUp = 100;

list($integerPart1, $integerPart2) = convertTo48bit($topUp);

$packedInteger = pack("vV", $integerPart1, $integerPart2);

echo json_encode($packedInteger) . "\n";

function convertTo48bit($integer)
{
    // convert to binary and pad to 48 bits
    $bin48bit = sprintf("%048b", $integer);

    // split into two and four byte chunks
    $twoBytes = substr($bin48bit, 0, 16);
    $fourBytes = substr($bin48bit, 16, 48);

    // convert each back to decimal
    $unsignedShort = bindec($twoBytes);
    $unsignedInteger = bindec($fourBytes);

    // return for use
    return array($unsignedShort, $unsignedInteger);
}