How would I create my own random function in any language?

function rand_string( $length ) {

$chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
return substr(str_shuffle($chars),0,$length);

}

echo rand_string(8);

This function using some predefine function. I want pure my own function which not use any predefine function.