<?php declare(strict_types=1); 

use Nette\PhpGenerator\ClassType;
use Nette\PhpGenerator\PhpFile;
use Nette\PhpGenerator\PhpNamespace;

$phpFile= new PhpFile();
$namespace = new PhpNamespace('FooBar');
$class = new ClassType('Foo');

$method = $class->addMethod('baz');
$method->setReturnType('FooBar\\Bar');
$method->setBody('return new Bar();');

$namespace->add($class);
$namespace->add(new ClassType('Bar'));

$phpFile->add($namespace);

echo $phpFile;

<?php

namespace FooBar;

class Foo
{
    public function baz(): Bar
    {
        return new Bar();
    }
}

class Bar
{
}