How works PHP Rand() function?

I wonder how PHP Rand() function works? I mean; How is this function randomising numbers without any rule or any other randomization function?

For example i have an array like this:

$numbers = array(0,1,2,3,4,5,6,7,8,9);

So how can i randomise this numbers without rand(), array_rand() etc..

Basic question is:

How randomization functions works?