7.1.0 rand() has been made an alias of mt_rand().
7.1.0 mt_rand()has been updated to use the fixed, correct, version of the Mersenne Twister algorithm. To fall back to the old behaviour, use mt_srand() with MT_RAND_PHP as the second paramter.

But for js : use Math.random or another ?

Was there a time when rand() was not using PHP?
I still wonder why manual examples stating {floor(rand(1,6.9999))}. The purpose of floor is unclear when rand() output is always integer. The 6.9999 as a parameter for php rand() is not needed.
https://manual.limesurvey.org/Expression_Manager_Sample_Surveys#Randomly_Ask_One_Question_Per_Group

The rand() JS for rand() PHP was this, right?

function rand (min, max) {
var args = arguments.length;
if (args === 0) {
min = 0;
max = 32768;
}
return Math.floor( Math.random() * (max - min + 1) ) + min;
}

MT implementations for JS are available. But choosing the right one might be a bigger task.

I'm really surprised that no rand() compatibility is maintained for PHP 7.1.
http://php.net/manual/en/migration71.incompatible.php#migration71.incompatible.rand-srand-aliases