Ordered arrays are awesome when we have data that lends itself to being collected into an ordered (indexed) list. But data can be collected and organized in lots of ways.

Imagine we wanted a data structure to hold a bunch of information about ourself: our name, age, email address, birthday, social security number, and favorite food. We want all of these pieces of data in a single collection but not necessarily in any particular order. And it doesn’t make much sense to access these data items using indices, like in an ordered array. It would be hard to remember, for example, that index 0 corresponds to our name.

Time to meet another fundamental concept in computer science—the map. A map associates keys with values. A key is a string or integer that we use to access a value (of any type). We could create a map with a key "fullname" that points to a value of "Aisle Nevertell"; this is much more understandable than associating the name with index 0. Whenever we need to access a value, we’ll be able to use the associated key to find it.

The PHP array type that we’ve been working with is actually implemented as a map! In a PHP ordered array, the index locations are the keys. But the PHP array type also enables us to build more traditional map-like structures where we assign meaningful keys to values (as opposed to indices). We call data structures like this associative arrays.

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