/!\ Spoil advent of code

day7.php

<?php

$i = file('i');
$z = [];
foreach ($i as $s) {
        $r = explode(' -> ', $s);
        $z[trim($r[1])] = trim($r[0]);
}

/* PART 2 (REMOVE THIS LINE FOR PART 1) */
$z['b'] = 46065;

function resolve($op, &$z) {
        $ops = explode(' ', $op);
        if (count($ops) === 1) {
                if (isset($z[$ops[0]]) && !is_numeric($ops[0])) {
                        return $z[$ops[0]] = (int) resolve($z[$ops[0]], $z);
                }

                return $z[$ops[0]] = (int) $ops[0];
        }

        if (count($ops) === 2) {
                return $z[$ops[1]] = ~ (int) resolve($ops[1], $z);
        }

        $a = resolve($ops[0], $z);
        $b = resolve($ops[2], $z);

        switch ($ops[1]) {
        case 'AND':
                $r = $a & $b;
                break;
        case 'OR':
                $r = $a | $b;
                break;
        case 'LSHIFT':
                $r = $a << $b;
                break;
        case 'RSHIFT':
                $r = $a >> $b;
                break;
        default:
                throw new Exception('Unknown op: ' . $ops[1]);
        }

        return (int) $r;
}

echo resolve($z['a'], $z);