Probability function

How can I create a function of probability, choosing between two strings with a probability range between 1/3, 1/2, 1/1, 2/1, 3/1

My value comes from a slider in a range between -2 and 2

this is my code:

$out = array('str1','str2');
$out = $out[array_rand($out)];

I want something like this:

$val = 0; //-2=1/3, -1=1/2, 0=1/1, 1=2/1, 2=3/1 

function random_str($val){
  ...
  retunt $myRandomValue;
}