PHP Cookbook

4.20. Randomizing an Array

Problem

You want to scramble the elements of an array in a random order.

Solution

If you’re running PHP 4.3 or above, use shuffle( ) :

shuffle($array);

If you’re running an earlier version, use the pc_array_shuffle( ) function shown in Example 4-4.

Example 4-4. pc_array_shuffle( )

function pc_array_shuffle($array) {
    $i = count($array);

    while(--$i) {
        $j = mt_rand(0, $i);

        if ($i != $j) {
            // swap elements
            $tmp = $array[$j];
            $array[$j] = $array[$i];
            $array[$i] = $tmp;
        }
    }

    return $array;
}

Here’s an example:

$cards = range(1,52); // deal out 52 "cards"
$cards = pc_array_shuffle($cards);

Discussion

There’s already a shuffle( ) function in PHP to shuffle arrays, but as of PHP 4.2.2, it doesn’t do its job correctly. The built-in shuffling algorithm tends to favor certain permutations more than others. Elements end up looking randomized, but since each element doesn’t have the same chance of ending up in each position, it’s not a true shuffle. This is fixed in PHP 4.3.

pc_array_shuffle( ) , known as the Fisher-Yates shuffle, equally distributes the elements throughout the array. Use it if you run a version of PHP earlier than 4.3. Unlike shuffle( ), this function returns the scrambled array instead of modifying it in-place. It also requires a tightly packed array with integer keys.

See Also

Recipe 4.22 for a function that simulates shuffling a deck of cards; documentation on shuffle( ) at http://www.php.net/shuffle.