Programming PHP

Chapter CHAPTER 5 5
Arrays
As we discussed in Chapter 2, PHP supports both scalar and compound data types.
In this chapter, we’ll discuss one of the compound types: arrays. An array is a collection
of data values, organized as an ordered collection of key-value pairs.
This chapter talks about creating an array, adding and removing elements from an
array, and looping over the contents of an array. There are many built-in functions
that work with arrays in PHP, because arrays are very common and useful. For example,
if you want to send email to more than one email address, you’ll store the email
addresses in an array and then loop through the array, sending the message to the
current email address. Also, if you have a form that permits multiple selections, the
items the user selected are returned in an array.
Indexed Versus Associative Arrays
There are two kinds of arrays in PHP: indexed and associative. The keys of an
indexed array are integers, beginning at 0. Indexed arrays are used when you identify
things by their position. Associative arrays have strings as keys and behave more like
two-column tables. The first column is the key, which is used to access the value.
PHP internally stores all arrays as associative arrays, so the only difference between
associative and indexed arrays is what the keys happen to be. Some array features are
provided mainly for use with indexed arrays, because they assume that you have or
want keys that are consecutive integers beginning at 0. In both cases, the keys are
unique—that is, you can’t have two elements with the same key, regardless of
whether the key is a string or an integer.
PHP arrays have an internal order to their elements that is independent of the keys
and values, and there are functions that you can use to traverse the arrays based on
this internal order. The order is normally that in which values were inserted into the
array, but the sorting functions described later let you change the order to one based
on keys, values, or anything else you choose.
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