Constant PHP_RUNTIME 

pub const PHP_RUNTIME: &str = r#"<?php
// OxiLean PHP Runtime
// Auto-generated — do not edit.

declare(strict_types=1);

namespace OxiLean\Runtime;

/** Represents a lazy thunk (unevaluated computation). */
final class Thunk
{
    private mixed $value = null;
    private bool $evaluated = false;
    /** @var callable */
    private $thunk;

    public function __construct(callable $thunk)
    {
        $this->thunk = $thunk;
    }

    public function force(): mixed
    {
        if (!$this->evaluated) {
            $this->value = ($this->thunk)();
            $this->evaluated = true;
        }
        return $this->value;
    }
}

/** Represents an OxiLean product (pair/tuple). */
final readonly class Prod
{
    public function __construct(
        public readonly mixed $fst,
        public readonly mixed $snd,
    ) {}
}

/** Represents an OxiLean sum (Either). */
abstract class Sum
{
    final public static function inl(mixed $val): self
    {
        return new class($val) extends Sum {
            public function __construct(public readonly mixed $val) {}
        };
    }
    final public static function inr(mixed $val): self
    {
        return new class($val) extends Sum {
            public function __construct(public readonly mixed $val) {}
        };
    }
}

/** Nat operations. */
final class Nat
{
    public static function succ(int $n): int { return $n + 1; }
    public static function pred(int $n): int { return max(0, $n - 1); }
    public static function add(int $a, int $b): int { return $a + $b; }
    public static function mul(int $a, int $b): int { return $a * $b; }
    public static function sub(int $a, int $b): int { return max(0, $a - $b); }
}
"#;

Minimal OxiLean PHP runtime header embedded in emitted files.