根据奖品的权重值， 实现概率性随机抽取的具体实现

1、初始数据：
权重越大，抽取的几率越高
[奖品1， 权重 5], [ 奖品2， 权重6]， [ 奖品3， 权重 7]， [ 奖品4， 权重2]
2、处理步骤：
1）N = 5 + 6 + 7 + 2 = 20
2）然后取1-N的随机数M
3）界定各 奖品的权重范围值 奖品 1 ： 1-5 ; 奖品2 ： 6-11； 奖品3： 12-18； 奖品4： 19-20
4) 如果M在某个奖品的权重范围值内，标识这个奖品被抽取到

 
<?php
/**
 * 奖品
 */
class Prize {
    # ID
    public $id = null;
    # 权重
    public $weight = null;
    # 奖品名
    public $name = null;
  
    # 权重范围区间起始值
    protected $start = 0;
    # 权重范围区间结束值
    protected $end = 0;
  
    public function __construct($id, $weight, $name) {
        if (!$id) {
            throw new Exception('奖品ID为空.');
        }
        $this->id = $id;
        $this->weight = $weight ? $weight : 0;
        $this->name = $name ? $name : '随机奖品' . $id;
    }
  
    # id
    public function getId() {
        return $this->id;
    }
  
    # 权重
    public function getWeight() {
        return $this->weight;
    }
  
    # 设置权重范围区间
    public function setRange($start, $end) {
        $this->start = $start;
        $this->end = $end;
    }
  
    # 判断随机数是否在权重范围区间
    public function inRange($num) {
        return ($num >= $this->start) && ($num <= $this->end);
    }
}
  
/**
 * 奖品池
 */
class PrizePoll implements IteratorAggregate, Countable {
    # 奖品集
    protected $items = array();
  
    # 加入奖品
    public function addItem(Prize $item) {
        $this->items[$item->getId()] = $item;
        return $this;
    }
  
    # 删除奖品
    public function removeItem($itemId) {
        if (isset($this->items[$itemId])) {
            unset($this->items[$itemId]);
        }
        return $this;
    }
  
    # 更新奖品
    public function updateItem(Prize $item) {
        if (isset($this->items[$item->getId()])) {
            $this->items[$item->getId()] = $item;
        }
        return $this;
    }
  
    # 获取所有奖品
    public function getItems() {
        return $this->items;
    }
  
    # 所有所有可用奖品(如果权重为0，说明这个奖品永远不可能抽到)
    public function getVisibleItems() {
        $items = array();
        foreach ($this->items as $item) {
            if ($item->getWeight()) {
                $items[$item->getId()] = $item;
            }
        }
        return $items;
    }
  
    # Countable::count
    public function count() {
        return count($this->items);
    }
  
    # IteratorAggregate::getIterator()
    public function getIterator() {
        return new ArrayIterator($this->items);
    }
}
  
/**
 * 简单的抽奖类
 */
class SimpleTurn {
    # 奖池
    protected $poll = null;
      
    public function __construct(PrizePoll $poll) {
        if ($poll) {
            $this->setPoll($poll);
        }
    }
  
    # 抽奖
    public function run(PrizePoll $poll) {
        $poll = $poll ? $poll : $this->poll;
        if ( ! $poll) {
            throw new Exception('奖池未初始化');
        }
  
        if ($poll->count() <= 0) {
            throw new Exception('奖池为空');
        }
  
        $items = $poll->getVisibleItems();
        if (count($items) <= 0) {
            throw new Exception('奖池为空');
        }
  
        $sum = 0;
        foreach ($items as $item) {
            $start = $sum + 1;
            $sum += $item->getWeight();
            $end = $sum;
  
            # 设置奖品的权重范围区间
            $item->setRange($start, $end);
        }
  
        # 随机数
        $rand = $this->getRandNum(1, $sum);
  
        # 区间段判断
        foreach ($items as $item) {
            if ($item->inRange($rand)) {
                return $item;
            }
        }
        return null;
    }
  
    # 获取随机数
    public function getRandNum($min, $max) {
        return mt_rand($min ? $min : 1, $max);
    }
  
    # 设置奖池
    public function setPoll(PrizePoll $poll) {
        $this->poll = $poll;
    }
}
  
# 示例
try {
    $prizePoll = new PrizePoll();
    $prizePoll->addItem(new Prize(1, 5))
        ->addItem(new Prize(2, 6))
        ->addItem(new Prize(3, 7))
        ->addItem(new Prize(4, 2));
  
    $turn = new SimpleTurn($prizePoll);
    $prize = $turn->run();
    var_dump($prize);
} catch (Exception $e) {
    print_r($e);
}