根据奖品的权重值， 实现概率性随机抽取的具体实现

<?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);
}