PHP框架中的一个奇怪的功能Slim

Question

以下函数normalizeKey，输入$key，返回$key，$key如何从sting转换为混合？

namespace Slim\Helper;

class Set implements \ArrayAccess, \Countable, \IteratorAggregate
{
    ....

    /**
     * Normalize data key
     *
     * Used to transform data key into the necessary
     * key format for this set. Used in subclasses
     * like \Slim\Http\Headers.
     *
     * @param  string $key The data key
     * @return mixed       The transformed/normalized data key
     */
    protected function normalizeKey($key)
    {
        return $key;
    }
    ....
}