PHP Cookbook, 3rd Edition

Chapter 4. Arrays

Introduction

Arrays are lists: lists of people, lists of sizes, lists of books. To store a group of related items in a variable, use an array. Like a list on a piece of paper, the elements in an array have an order. Usually, each new item comes after the last entry in the array, but just as you can wedge a new entry between a pair of lines already in a paper list, you can do the same with arrays in PHP.

Most languages have numerical arrays (sometimes referred to just as arrays). In a numerical array, if you want to find an entry, you need to know its position within the array, known as an index. Positions are identified by numbers: they start at 0 and work upward one by one.

In some languages, there is also another type of array: an associative array, also known as a hash or a map or a dictionary. In an associative array, indexes aren’t integers, but strings. So in a numerical array of US presidents, “Abraham Lincoln” might have index 16; in the associative-array version, the index might be “Honest.” However, whereas numerical arrays have a strict ordering imposed by their keys, associative arrays frequently make no guarantees about the key ordering. Elements are added in a certain order, but there’s no way to determine the order later.

When a language has both numerical and associative arrays, usually the numerical array $presidents and the associative array $presidents are distinct arrays. Each array type has a specific behavior, and you need to operate on it accordingly. ...