Binary GCD in PHP

<?php
declare(strict_types=1);
// Binary GCD in PHP
//
// PURPOSE
// Binary GCD replaces general remainders with parity tests, division by two, and
// subtraction.
//
// METHOD
// Remove common factors of two, make the first operand odd, repeatedly make the second odd
// and subtract the smaller odd operand, then restore the common power of two.
//
// COMPLEXITY
// O(log(max(a,b))) iterations under fixed-width arithmetic, O(1) space.
//
// CONTRACT
// Nonnegative integers. gcd(0,0)=0.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// PHP 8.1+ on a 64-bit runtime: packed arrays, strict scalar types, read-only inputs by
// value using copy-on-write, references only for outputs, and checked 32-bit arithmetic.
// Invalid input and overflow throw exceptions. PHP arrays have higher overhead than native
// packed buffers in C/Java.
//
// EXAMPLE
// Expected return: 6.
// Run: php example.php
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

function alg_require(bool $valid): void { if (!$valid) throw new InvalidArgumentException("invalid algorithm input"); }
function alg_checked(int|float $v): int { if ($v < -2147483648 || $v > 2147483647 || floor($v) != $v) throw new OverflowException("32-bit result overflow"); return (int)$v; }
function alg_add(int $a,int $b): int { return alg_checked($a+$b); }
function alg_sub(int $a,int $b): int { return alg_checked($a-$b); }
function alg_mul(int $a,int $b): int { return alg_checked($a*$b); }
function alg_size(int $n): int { alg_require($n>=0 && $n<=4000000); return $n; }

function binary_gcd(int $a, int $b): int {
    alg_require((($a >= 0) && ($b >= 0)));
    if (($a == 0)) {
        return $b;
    }
    if (($b == 0)) {
        return $a;
    }
    $factor = 1;
    $temp = 0;
    while (((($a % 2) == 0) && (($b % 2) == 0))) {
        $a = intdiv($a, 2);
        $b = intdiv($b, 2);
        $factor = alg_mul($factor, 2);
    }
    while ((($a % 2) == 0)) {
        $a = intdiv($a, 2);
    }
    while (($b != 0)) {
        while ((($b % 2) == 0)) {
            $b = intdiv($b, 2);
        }
        if (($a > $b)) {
            $temp = $a;
            $a = $b;
            $b = $temp;
        }
        $b = alg_sub($b, $a);
    }
    return alg_mul($a, $factor);
}

$a = 48;
$b = 18;
$result = binary_gcd($a, $b);
echo $result, PHP_EOL;