php字符串送分题

$a='331612_1,331495_1,331461_2,331559_1,333080_1,333555_1,331448_1,331618_1,333388_1,333408_1,327937_1,331615_1,331499_1';
怎么获取_后面的数字，并且将字符串_后面的数字相加。

先来一个：
PHP code

$a='331612_1,331495_1,331461_2,331559_1,333080_1,333555_1,331448_1,331618_1,333388_1,333408_1,327937_1,';
preg_match_all('/_(\d+)/U',$a,$arr);
echo array_sum($arr[1]);