readonly.php

<?php

class X {
    private int $a = 0;

    public function incr() {
        $this->a++;
    }
    
    public function a() {
        return $this->a;
    }
}

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

    public function b() {
        $this->x->incr();
        return $this->x->a();
    }
}

$a = new A(new X());
var_dump($a->b());

The result is 1