在 PHP 中在实例化自己的对象的函数中使用模拟对象

I have been looking into how to add unit testing coverage to a large, existing codebase written in PHP. Many functions in both static and instantiable classes make a call to a library or instantiate an object in order to obtain connections to memcache and the database. They typically look something like this:

public function getSomeData() {
    $key = "SomeMemcacheKey";
    $cache = get_memcache();

    $results = $cache->get($key);
    if (!$results) {
        $database = new DatabaseObject();
        $sql = "SELECT * from someDatabase.someTable";
        $results = $database->query($sql);

        $cache->set($key, $results);
    }

    return $results;
}

我和我的同事目前正在尝试通过 PHPUnit 为我们正在编写的一些新类实现覆盖.我试图找到一种方法，以隔离方式为我们现有代码库中的函数创建单元测试，这些函数类似于上面的伪代码，但没有成功.