Виталий Хоменко

/**
* @var mixed[]
*/
protected $items = [];

/**
 * @property SomeClass[] $items
 */
class SomeCollection extends IterableCollection
// ...

public function doSomeWork()
{
    /** @var SomeClass[] $items */
    $items =  $this->items;

    $items[0]->someMethod();
}

/**
 * @return SomeClass[]
 */
protected function getItems ()
{
    return $this->items;
}
 
public function doSomeWork()
{
    $this->getItems()[0]->someMethod();
}