<?php

class Foo {
    public function __construct($prop) {
        $this->prop = $prop;
    }
    
    public function __toString() {
        return $this->prop;
    }
}

class MyClass
{
    public function __construct()
    {
        $this->prop = new Foo("test");
    }

    public function test()
    {
        $class = new class($this->prop) {

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

            public function run() 
            {
                $this->prop->prop = "Teste na classe anônima";
                echo $this->prop, PHP_EOL;
            }

        };
        
        $class->run();
    }
}

$obj = new MyClass();
$obj->test();

echo $obj->prop, PHP_EOL;