弹出框div页面居中/遮罩

function _getposition(){  
	var p = {};  
	var scrollWidth, scrollHeight;  
	if(window.innerHeight && window.scrollMaxY){  
		scrollWidth = window.innerWidth + window.scrollMaxX;  
		scrollHeight = window.innerHeight + window.scrollMaxY;  
	}else if(document.body.scrollHeight>document.body.offsetHeight){  
		scrollWidth = document.body.scrollWidth;  
		scrollHeight = document.body.scrollHeight;  
	}else{  
		scrollWidth = document.body.offsetWidth;  
		scrollHeight = document.body.offsetHeight;  
	}  
	if(self.innerHeight){  
		if(document.documentElement.clientWidth){  
			p.windowWidth = document.documentElement.clientWidth;  
		}else{  
			p.windowWidth = self.innerWidth;  
		}  
		p.windowHeight = self.innerHeight;  
	}else if(document.documentElement && document.documentElement.clientHeight){  
		p.windowWidth = document.documentElement.clientWidth;  
		p.windowHeight = document.documentElement.clientHeight;  
	}else if(document.body){  
		p.windowWidth = document.body.clientWidth;  
		p.windowHeight = document.body.clientHeight;  
	}  
	if(scrollWidth < p.windowWidth){  
		p.width = scrollWidth;  
	}else{  
		p.width = p.windowWidth;  
	}  
	if(scrollHeight < p.windowHeight){  
		p.height = scrollHeight;  
	}else{  
		p.height = p.windowHeight;  
	}  
	p.windowWidth = Math.max(p.windowWidth, scrollWidth);  
	p.windowHeight = Math.max(p.windowHeight, scrollHeight);  
  
	if(typeof(window.pageXOffset) == "number"){  
		p.left = window.pageXOffset;  
	}else if(document.documentElement && document.documentElement.scrollLeft){  
		p.left = document.documentElement.scrollLeft;  
	}else if(document.body){  
		p.left = document.body.scrollLeft;  
	}else if(window.scrollX){  
		p.left = window.scrollX;  
	}  
  
	if(typeof(window.pageYOffset) == "number"){  
		p.top = window.pageYOffset;  
	}else if(document.documentElement && document.documentElement.scrollTop){  
		p.top = document.documentElement.scrollTop;  
	}else if(document.body){  
		p.top = document.body.scrollTop;  
	}else if(window.scrollY){  
		p.top = window.scrollY;  
	}  
	return p;  
}  
var p = _getposition();  
var left = p.left + ((p.width - $("#ddl_egg_msg_div").width()) / 2);  
var top = p.top + ((p.height - $("#ddl_egg_msg_div").height()) / 2);                          
$("#ddl_egg_msg_div").css({left:left,top:top});  
$("#coverdiv").width(p.width).height(p.windowHeight).show();//整页遮罩  

 div出现在页面的随机位置

var sHeight = jQuery(document.body).outerHeight(true) - 100;  
var sWidth = jQuery(document.body).outerWidth(true) - 100;  
  
var div_top  = GetRandomNum(0,sHeight);  
var div_left = GetRandomNum(0,sWidth);  
  
jQuery("#div").css("position","absolute");  
jQuery("#div").css("z-index",100000);  
jQuery("#div").css("top",div_top);  
jQuery("#div").css("left",div_left);  
  
function GetRandomNum(Min,Max)  
{  
	var Range = Max - Min;  
	var Rand = Math.random();  
	return(Min + Math.round(Rand * Range));  
}  

该算法简单使用，并发访问性能非常好。随机概率

function get_rand($proArr)
{
    $result = '';
    //概率数组的总概率精度 
    $proSum = array_sum($proArr);
    //概率数组循环 
    foreach ($proArr as $key => $proCur) {
        $randNum = mt_rand(1, $proSum);
        if ($randNum <= $proCur) {
            $result = $key;
            break;
        } else {
            $proSum -= $proCur;
        }
    }
    unset($proArr);
    return $result;
}

调用

//本次抽奖的奖项信息，必须按照从大到小的顺序进行填写，id为奖次，gift为中奖信息,v为中奖概率必须是整数,num为奖品数量
//需要注意的是，该处也必须包含不中奖的信息，概率从小到大进行排序
$gift_arr = array(
    '0' => array('id' => 1, 'gift' => '44元购买1G/年空间', 'v' => 1, 'num' => 1),
    '1' => array('id' => 2, 'gift' => '55元购买1G/年空间', 'v' => 2, 'num' => 2),
    '2' => array('id' => 3, 'gift' => '66元购买1G/年空间', 'v' => 5, 'num' => 2),
    '3' => array('id' => 4, 'gift' => '77元购买1G/年空间', 'v' => 10, 'num' => 3),
    '4' => array('id' => 5, 'gift' => '88元购买1G/年空间', 'v' => 15, 'num' => 4),
    '5' => array('id' => 6, 'gift' => '99元购买1G/年空间', 'v' => 67, 'num' => 10)
);

foreach ($gift_arr as $val) {
    $arr[$val['id']] = $val['v'];
}
//$rid中奖的序列号码
$rid = $this->get_rand($arr); //根据概率获取奖项id
$str = $gift_arr[$rid - 1]['gift']; //中奖项

 中奖到达上限后，中的奖项换成不中奖，中的奖项发完换成不中奖