In Place Array Reversal in PHP

<?php
declare(strict_types=1);
// In Place Array Reversal in PHP
//
// PURPOSE
// Reversal exchanges symmetric positions until the two pointers meet. It is a building block
// for rotation and sequence manipulation.
//
// METHOD
// Swap the leftmost and rightmost unprocessed values, then move both pointers inward.
// Everything outside the pointers is already in its final reversed position.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space.
//
// CONTRACT
// Reverses n elements in place and returns 0. Empty arrays and single elements are valid.
// 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: 0.
// Expected a after call: [3, 2, 1].
// 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 reverse_array(array &$a): int {
    alg_require(array_is_list($a));
    $n = count($a);
    alg_require((($n >= 0) && ($n <= 1000000)));
    alg_require((count($a) == $n));
    $left = 0;
    $right = alg_sub($n, 1);
    $temp = 0;
    while (($left < $right)) {
        $temp = $a[$left];
        $a[$left] = $a[$right];
        $a[$right] = $temp;
        $left = alg_add($left, 1);
        $right = alg_sub($right, 1);
    }
    return 0;
}

$a = [1, 2, 3];
$result = reverse_array($a);
echo $result, PHP_EOL;
echo json_encode($a), PHP_EOL;
// Expected a: [3, 2, 1]