Immutable.php

<?php

class A
{
    private $a;
    
    public function __construct($a)
    {
        $this->a = $a;
    }
}

class B extends A
{
    /**
     * Use php's magic method to enforce immutability
     */
    public function __set($property, $value)
    {
        throw new RuntimeException(self::class . ' is immutable');
    }
}

$a = new A(1);
$b = new B(2);

$a->b = $b;
var_dump($a);

try {
    $b->c = 3;
} catch (RuntimeException $re) {
    var_dump($b);
}

http://3v4l.org/RC1Sk