Arrays

An array in PHP is actually an ordered map. A map is a type that maps values to keys. This type is optimized in several ways, so you can use it as a real array, or a list (vector), hashtable (which is an implementation of a map), dictionary, collection, stack, queue and probably more. Because you can have another PHP-array as a value, you can also quite easily simulate trees.

Explanation of those structures is beyond the scope of this manual, but you'll find at least one example for each of those structures. For more information about those structures, we refer you to external literature about this broad topic.

Syntax

Specifying with array()

An array can be created by the array() language-construct. It takes a certain number of comma-separated key => value pairs.

A key is either a nonnegative integer or a string. If a key is the standard representation of a non-negative integer, it will be interpreted as such (i.e. '8' will be interpreted as 8, while '08' will be interpreted as '08').

A value can be anything.

If you omit a key, the maximum of the integer-indices is taken, and the new key will be that maximum + 1. If no integer-indices exist yet, the key will be 0 (zero). If you specify a key that already has a value assigned to it, that value will be overwritten.

array( [key =>] value
     , ...
     )
// key is either string or nonnegative integer
// value can be anything

Useful functions

There are quite some useful function for working with arrays, see the array-functions section.

The foreach control structure exists specificly for arrays. It provides an easy way to traverse an array.

Array do's and don'ts

Why is $foo[bar] wrong?

You might have seen the following syntax in old scripts:

This is wrong, but it works. Then, why is it wrong? The reason is that, as stated in the syntax section, there must be an expression between the square brackets ('[' and ']'). That means that you can write things like this: This is an example of using a function return value as the array index. PHP knows also about constants, and you may have seen the E_* before. Note that E_ERROR is also a valid identifier, just like bar in the first example. But the last example is in fact the same as writing: because E_ERROR equals 1, etc.

Then, how is it possible that $foo[bar] works? It works, because bar is due to its syntax expected to be a constant expression. However, in this case no constant with the name bar exists. PHP now assumes that you meant bar literally, as the string "bar", but that you forgot to write the quotes.

Examples

The array type in PHP is very versatile, so here will be some examples to show you the full power of arrays.

Note that it is currently not possible to change the values of the array directly in such a loop. A workaround is the following:

This example creates a one-based array.

Arrays are ordered. You can also change the order using various sorting-functions. See array-functions for more information.

Because the value of an array can be everything, it can also be another array. This way you can make recursive and multi-dimensional arrays.