PHP Cookbook

4.17. Sorting an Array by a Computable Field

Problem

You want to define your own sorting routine.

Solution

Use usort( ) in combination with a custom comparison function:

// sort in reverse natural order
function natrsort($a, $b) {
    return strnatcmp($b, $a);
}

$tests = array('test1.php', 'test10.php', 'test11.php', 'test2.php');
usort($tests, 'natrsort');

Discussion

The comparison function must return a value greater that 0 if $a > $b, 0 if $a == $b, and a value less than 0 if $a < $b. To sort in reverse, do the opposite. The function in the Solution, strnatcmp( ) , obeys those rules.

To reverse the sort, instead of multiplying the return value of strnatcmp($a, $b) by -1, switch the order of the arguments to strnatcmp($b, $a).

The sort function doesn’t need to be a wrapper for an existing sort. For instance, the pc_date_sort( ) function, shown in Example 4-2, shows how to sort dates.

Example 4-2. pc_date_sort( )

// expects dates in the form of "MM/DD/YYYY"
function pc_date_sort($a, $b) {
    list($a_month, $a_day, $a_year) = explode('/', $a);
    list($b_month, $b_day, $b_year) = explode('/', $b);

    if ($a_year  > $b_year ) return  1;
    if ($a_year  < $b_year ) return -1;

    if ($a_month > $b_month) return  1;
    if ($a_month < $b_month) return -1;

    if ($a_day   > $b_day  ) return  1;
    if ($a_day   < $b_day  ) return -1;

    return 0;
}

$dates = array('12/14/2000', '08/10/2001', '08/07/1999');
usort($dates, 'pc_date_sort');

While sorting, usort( ) frequently recomputes the sort function’s return values each time it’s ...