AnonymousClass.php

<?php

function foo(int $initialBaz)
{
    return new class ($initialBaz) {
        public function __construct(private int $baz) {}

        public function getBaz(): int
        {
            return $this->baz;
        }

        public function setBaz(int $newValue): void
        {
            $this->baz = $newValue;
        }
    };
}

$fooObject = foo(3);

$currentValue = $fooObject->getBaz(); // 3
$fooObject->setBaz(5);
$newValue = $fooObject->getBaz(); // 5