What is the PHP sort array by key?

Sorting is an algorithmic approach to ordering items. All linear data structures can be arbitrarily ordered: the PHP array is no exception. The PHP array—a hash table—is perhaps the language's quintessential composite data structure. It is a dynamic artifact that can be used as a traditional numeric array, associative array, multidimensional array, or any combination of the options mentioned above. Much like the rest of the language, PHP's array structure has evolved steadily through several iterations to offer competitive performance.

As with most array implementations, the items in PHP arrays have indexes with which it is possible to access their values. The indexes are either of a numeric or string type. Sorting an array by key, therefore, is a task that involves ordering a PHP array by evaluating the properties of its keys. PHP offers functions that make sorting arrays by key efficient.

The ksort and krsort functions make it possible to sort array keys, and transitively, array elements, in ascending and descending order respectively. For arrays with string keys, the ksort and krsort functions produce respective alphabetical and reverse-alphabetical key orderings. With a custom function like custom_sort in the snippet to follow, it is possible to abstract the use of the aforestated array key sorting functions.