根据奖品概率计算获取的奖品想法

$arr = array(
    'a1' => 1,
    'a2' => 2,
    'a3' => 3,
    'a4' => 4
);
$set = array();
for($i = 0 ;$i<=100000;$i++){
    $set[] = getPrizeKey($arr);
}
var_dump(array_count_values($set));

function getPrizeKey($weightMap){
    $sum = array_sum($weightMap);
    $low = 0;
    $rand = rand(1,$sum);
    foreach($weightMap as $key => $weight){
        if($rand>$low && $rand<= $low+$weight){
            return $key;
        }
        $low = $low+$weight;
    }
}