//key：奖项key，value：中奖的概率占比或者份数
$array = ['key1' => 20, 'key2' => 40, 'key3' => 30, 'key5' => 10];

//return：返回中奖的key
$fun = function ($array) {
    asort($array);//概率大小排序，分配区间，0-10-20-30-40;
    $randNum = mt_rand(1, array_sum($array));//随机数
    //print_r($randNum . PHP_EOL);

    //区间判断
    $start = 0;
    foreach ($array as $key => $item) {
        //构造区间数：0-10-20-30-40;
        $start += $item;
        //print_r('start:' . $start . ' key:' . $key . PHP_EOL);
        if ($randNum <= $start) {
            return $key;
        }
    }
};
var_dump($fun($array));