Arrays, references and generators: guess the output

<?php

$a = [1, 2];
$b = [3, 4, 5];

echo implode(",", $a + $b), " | ", implode(",", array_merge($a, $b));

<?php

$items = [1, 2, 3];

foreach ($items as &$item) {
}

foreach ($items as $item) {
}

echo implode(",", $items);

<?php

function gen()
{
    echo "A";
    yield 1;
    echo "B";
    yield 2;
    echo "C";
}

foreach (gen() as $value) {
    echo $value;
}

<?php

class Model
{
    public static function viaStatic(): static
    {
        return new static;
    }

    public static function viaSelf(): self
    {
        return new self;
    }
}

class User extends Model {}

echo get_class(User::viaStatic()), " ", get_class(User::viaSelf());

<?php

$data = ["key" => null];

echo isset($data["key"]) ? "set" : "not set";
echo " / ";
echo array_key_exists("key", $data) ? "exists" : "missing";