vo.php

<?php

abstract class ValueObject
{
	private $data = [];
	
	final public function __construct()
	{
		assert((function () {
			foreach ((new ReflectionClass(static::class))->getProperties() as $p) {
				$p = $p->getName();
				$this->data[$p] = $this->$p;
				unset($this->$p);
			}
		})() || true);
	}
	
	final public function __get($p)
	{
		if (!array_key_exists($p, $this->data)) {
			$className = static::class;
			throw new \Error("Instance of $className does not have a property named $p");
		}
		
		return $this->data[$p];
	}
	
	final public function __set($p, $v)
	{
		if (!array_key_exists($p, $this->data)) {
			$className = static::class;
			throw new \Error("Instance of $className does not have a property named $p");
		}
		
		ob_start();
		debug_zval_dump($this);
		preg_match('~refcount\((\d+)\)~', ob_get_clean(), $matches);
		$refCount = $matches[1] - 3;
		
		if ($refCount > 1) {
			$className = static::class;
			throw new \Error("Can't mutate instance of $className, it has more than one reference. Did you assign it without cloning?");
		}
		
		$this->data[$p] = $v;
	}
}

class Something extends ValueObject
{
	public $foo;
}

$v = new Something();
$v->foo = "A";
$v2 = clone $v;
$v2->foo = "B";
var_dump($v->foo);
var_dump($v2->foo);