random_picker

gistfile1.php

<?php

function make_random_picker ($item_list,$weight_list) {
    $total_weight = array_sum($weight_list);
    $weighted_array = array_combine($item_list,$weight_list);
    arsort($weighted_array);

    return function () use (&$total_weight, &$weighted_array) {
        $p = 0;
        $r = mt_rand(1,$total_weight);
        foreach ($weighted_array as $key => $value) {
            $p += $value;
            if ($r <= $p) {
                return $key;
            }

        }
    };
}

$c = make_random_picker(
    ['db2','db1','db3'],
    [25,50,25]
);


$test = ['db1' => 0, 'db2' => 0, 'db3' => 0];
foreach(range(1,100000) as $i){
    $test[$c()]++;
}
print_r($test);

?>