php计算字符串中的数字之和

<?php
header('content-type:text/html;charset=utf-8');

$str='331612_1,331495_1,331461_1,331559_1,333080_1,333555_1,331448_1,331618_1,333388_1,333408_1,327937_1,331615_1,331499_1';
$str = explode(',',$str);
$sum=0;
foreach($str as $key=>$value){
	$str =strpos($value, '_');
	$a =substr($value,$str+1);
	$sum +=$a;
}

print_r($sum);