Question

I’m looking for a function that I can pass an array and a seed to in PHP and get back a “randomized” array. If I passed the same array and same seed again, I would get the same output.

I’ve tried this code

//sample array
$test = array(1,2,3,4,5,6);
//show the array
print_r($test);

//seed the random number generator
mt_srand('123');
//generate a random number based on that
echo mt_rand();
echo "\n";

//shuffle the array
shuffle($test);

//show the results
print_r($test);

But it does not seem to work. Any thoughts on the best way to do this?

This question dances around the issue but it’s old and nobody has provided an actual answer on how to do it: Can i randomize an array by providing a seed and get the same order? – “Yes” – but how?

Update

The answers so far work with PHP 5.1 and 5.3, but not 5.2. Just so happens the machine I want to run this on is using 5.2.

Can anyone give an example without using mt_rand? It is “broken” in php 5.2 because it will not give the same sequence of random numbers based off the same seed. See the php mt_rand page and the bug tracker to learn about this issue.

Answer 1

You can use array_multisort to order the array values by a second array of mt_rand values:

$arr = array(1,2,3,4,5,6);

mt_srand('123');
$order = array_map(create_function('$val', 'return mt_rand();'), range(1, count($arr)));
array_multisort($order, $arr);

var_dump($arr);

Here $order is an array of mt_rand values of the same length as $arr. array_multisort sorts the values of $order and orders the elements of $arr according to the order of the values of $order.