Add two integers modulo 2^32 on a 32-bit system

32bitadd.php

<?php

/**
 * Since integers in 32-bit PHP 5.x are signed, exclusively, this only goes up to 2147483647
 */
function add_32bit($intA, $intB)
{
    if (!is_int($intA) || !is_int($intB)) {
        return false;
    }
    if ($intA > 2147483647 || $intB > 2147483647) {
        return false;
    }
    $sum = 0;
    $carry = 0;
    $rem = 0;

    for ($i = 0; $i < 4; ++$i) {
        $oA = $intA >> ($i * 8);
        $oB = $intB >> ($i * 8);
        $pA = $oA & 0xff;
        $pB = $oB & 0xff;

        $tmp = ($pA + $pB + $carry);
        $rem = $tmp & 0xff;
        $carry = ($tmp - $rem) >> 8;
        $inc = ($rem << ($i * 8));
        $sum |= $inc & 0x7fffffff;
    }
    return $sum;
}

tests.php

<?php
include "32bitadd.php";
// Tests
var_dump( add_32bit(1000, 1001) );
var_dump( add_32bit(1000, 2000) );
var_dump( add_32bit(1, 1) );
var_dump( add_32bit(10000000, 10000000) );
var_dump( 2147483646, 1 );
var_dump( 2147483646, 2 );
var_dump( 2147483646, 3 );
var_dump( 2147483646, 4 );

http://3v4l.org/7AIOa