PHP 隨機數字按權重

$result = array();
for($i=0; $i<100; $i++){
$row = $this->roll(array('a'=>290,'b'=>400,'c'=>210, 'd'=>100, 'e'=>20)); //a=1-5 29% b=6-10 40% c=11-15 21% d=16-20 10% e=21-30 2%

switch ($row){
case 'a':
array_push($result,mt_rand(1, 5));
break;
case 'b':
array_push($result,mt_rand(6, 10));
break;
case 'c':
array_push($result,mt_rand(11, 15));
break;
case 'd':
array_push($result,mt_rand(16, 20));
break;
case 'e':
array_push($result,mt_rand(21, 40));
break;
}
}
echo "<pre>";
print_r($result);
return ;