生成有偏随机数

<?php
/* 大致思想就是：将剩余展示数目累加
比如：A:5，B:8，C:10
累加后：A:5，B:13,C:23
然后随即一个数,范围0~23
判断 随机数是否小于A或B或C，小于则成功 

效果：谁的剩余展示数目越多，被随机到的几率越高
*/
function pc_rand_weighted($numbers) {
    $total = 0;
    foreach ( $numbers as $number => $weight ) {
        $total += $weight;
        $distribution [$number] = $total;
    }
    $rand = mt_rand ( 0, $total - 1 );
    foreach ( $distribution as $number => $weights ) {
        if ($rand < $weights) {
            return $number;
        }
    }
}
$ads = array (
        'ford' => 12234,//广告客户，剩余展示数目
        'att' => 33424,
        'ibm' => 16823 
);

$ad = pc_rand_weighted ( $ads );
var_dump ( $ad );
?>