还是php的概率的有关问题，求解说

还是php的概率的问题，求解说
概率计算函数，网上搜了很多都是这个，不过没看得懂这函数，求解下过程是怎样的~
这数组是id和相应的概率。

Array

(

????[1] => 1

????[2] => 5

????[3] => 10

????[4] => 12

????[5] => 22

????[6] => 50

)

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 ($randNum 
????????????$result = $key; 

????????????break; 

????????} else { 

????????????$proSum -= $proCur; 

????????} 

????} 

????unset ($proArr); 

????return $result; 

}