Shuffle array with a repeatable/predictable result based on a seed integer

Question

I have 6 users and I want to shuffle an array for each user but with a particular logic.

I have array like this:

$a = array(1, 6, 8);

When shuffled it gives me these results:

shuffle($a); //array(8,6,1) or array(8,1,6) or ...

I want to shuffle the array for a specific user and have it be the same every time for that user:

• for user that has id equals 1, give array like this array(6,8,1) every time
• for user that has id equals 2, give array like this array(1,8,6) every time

In other words, I want to shuffle an array with private key!

Answer 1

If you provide a seed to the random number generator it will randomize the same way for the same seed (see the version differences below). So use the user id as the seed:

srand(1);
shuffle($a);

Output for 7.1.0 - 7.2.4

Array
(
    [0] => 1
    [1] => 8
    [2] => 6
)

Output for 4.3.0 - 5.6.30, 7.0.0 - 7.0.29

Array
(
    [0] => 6
    [1] => 1
    [2] => 8
)

Note: As of PHP 7.1.0, srand() has been made an alias of mt_srand().

This Example should always produce the same result.