In PHP, APC (Alternative PHP Cache) is a widely used caching system that helps improve script execution efficiency. By caching bytecode and PHP variables, APC reduces disk I/O and enhances response speed. It provides various APIs to access and manage cache data conveniently. One such interface is the APCUIterator class, which is used to iterate over all key-value pairs in the APC cache. The APCUIterator::key method specifically allows developers to retrieve the cache key currently pointed to by the iterator.

According to the official PHP documentation, the return value of the APCUIterator::key() method is a (string). This means that no matter what the initial format of the key in the cache is, the method will always convert it into a string before returning it.

In PHP, array operations are very common. Getting the last element of an array is usually a simple operation, but how do you get the key name of the last element at the same time? At this time, the two functions end() and key() can help us achieve this goal.

When filling an array with array_fill(), which value does the key of the array start from? How to determine the key value?