Programming PHP, 2nd Edition

Sorting

Sorting changes the internal order of elements in an array and optionally rewrites the keys to reflect this new order. For example, you might use sorting to arrange a list of scores from biggest to smallest, to alphabetise a list of names or to order a set of users based on how many messages they posted.

PHP provides three ways to sort arrays—sorting by keys, sorting by values without changing the keys, or sorting by values and then changing the keys. Each kind of sort can be done in ascending order, descending order, or an order defined by a user-defined function.

Sorting One Array at a Time

The functions provided by PHP to sort an array are shown in Table 5-1.

Table 5-1. PHP functions for sorting an array

Effect	Ascending	Descending	User-defined order
Sort array by values, then reassign indices starting with 0	sort( )	rsort( )	usort( )
Sort array by values	asort( )	arsort( )	uasort( )
Sort array by keys	ksort( )	krsort( )	uksort( )

The sort( ), rsort( ), and usort( ) functions are designed to work on indexed arrays because they assign new numeric keys to represent the ordering. They’re useful when you need to answer questions such as, “What are the top 10 scores?” and “Who’s the third person in alphabetical order?” The other sort functions can be used on indexed arrays, but you’ll only be able to access the sorted ordering by using traversal functions such as foreach and next.

To sort names into ascending alphabetical order, you’d use this:

 $names = array('cath', 'angela', ...