使用闭包就可以实现了

<?php

class Books{
    public $count = 1;
    private $sum = 2;
}

function getInitializer(Books $books)
{
    return \Closure::bind(function () use ($books) {
        $books->sum  = 5;
    }, null, Books::class);
}

$books = new Books();
call_user_func(getInitializer($books));
var_dump($books);