Weighted percentage between two links?

I'm working with two rotating links in a php redirector, is there a way to weight them in percentages?

For example: The first link will have a 70% chance or getting redirected and second 30%.

Code:

    <?php
$k = $_GET['sub'];

$aff[] = 'http://google.com';
$aff[] = 'http://yahoo.com';

srand ((double) microtime() * 1000000);
$random_number = rand(0,count($aff)-1);

$lol = ($aff[$random_number]);
$lal = $lol.$k;

header("Location: $lal");
?>

Note: There are similar questions but involve more than two variables.